Why is "Using 1,024 instead of 1,000 when converting between prefixes." a common mistake in Units of information?

Why it happens: 1,024 appears constantly in computing contexts and in what operating systems report, so it feels like the 'proper' computing value — and many textbooks still teach it. How to avoid it: AQA uses the **SI decimal prefixes**: 1 kB = **1,000 bytes**, and every step up the ladder is another factor of 1,000. The specification states that the powers-of-2 terms are **kibi** and **mebi** and that students do **not** need to know them, so 1,024 should never appear in an answer. Source: AQA 8525 specification 3.3.3 additional information states the SI prefixes refer to powers of 10 and that kibi/mebi are not required.

Why is "Confusing bits with bytes, or reading `Mb` as megabytes rather than megabits." a common mistake in Units of information?

Why it happens: The symbols differ only in capitalisation, which is easy to overlook when scanning a question quickly — and the words themselves sound almost identical when read. How to avoid it: Fix the symbols: **lower-case b is a bit, capital B is a byte**, and they differ by a factor of **8**. Before starting any calculation, underline whether the question is in bits or bytes. A 100 Mb/s connection delivers 12.5 MB/s. Source: AQA 8525 specification 3.3.3 states explicitly that b represents bit and B represents byte.

Why is "Forgetting to divide by 8 when a file-size calculation gives a result in bits but the question asks for bytes." a common mistake in Units of information?

Why it happens: The main calculation feels finished once the multiplication is done, and the unit conversion reads like an afterthought rather than part of the answer. How to avoid it: The bitmap and sound formulae both produce **bits**. Always finish by converting: **÷8** for bytes, then **÷1,000** for kB. Sense-check the result — a photograph of 24 MB when you expected a few MB is exactly a factor of eight out. Source: AQA 8525 specification 3.3.6 gives both forms of the bitmap formula — in bits and in bytes — precisely because the conversion is required.

Why is "Multiplying when you should divide, or vice versa, when moving between prefixes." a common mistake in Units of information?

Why it happens: Both operations use the same factor of 1,000, so the arithmetic feels symmetrical and the direction is easy to reverse under time pressure. How to avoid it: Going **up** the ladder to a larger unit, the **number gets smaller**, so you **divide**. Going **down** to a smaller unit, the number gets larger, so you **multiply**. Sense-check: 5,000 bytes is 5 kB, not 5,000,000 kB. Source: Direction errors produce answers a million times out and are readily caught by a quick plausibility check.

Why is "Comparing quantities in different units by eye, without converting them first." a common mistake in Units of information?

Why it happens: A larger prefix intuitively suggests a larger quantity, so 0.5 GB *looks* bigger than 800 MB and the comparison seems obvious enough to skip. How to avoid it: Always **convert everything to one unit** before comparing — usually the smallest, to avoid decimals. 0.5 GB is 500 MB, which is genuinely **smaller** than 800 MB. Mark schemes credit the conversions, not just the conclusion. Source: AQA 8525 specification 3.3.3 requires students to be able to compare quantities of bytes using the prefixes.

Why is "Saying a byte can store values up to 256, or that it holds 255 different values." a common mistake in Units of information?

Why it happens: The two numbers are both firmly associated with 'one byte' and differ by only one, so under pressure the wrong one is recalled. How to avoid it: A byte holds **256 different values**, running from **0 to 255** — the count includes zero, so the **maximum value** is one less than the count. The same reasoning gives 4 bits sixteen values from 0 to 15. Source: This mirrors the counting distinction in 3.3.1 and 3.3.2, where the specification's stated maximum is 255.

AQA GCSE Computer Science (8525)

Units of information

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Detailed Study Notes

Detailed notes on Fundamentals of Data Representation for AQA GCSE Computer Science, covering key concepts, explanations, examples, and exam-focused revision points.

Units of Information — AQA GCSE Computer Science 8525 (3.3.3)

A short subtopic with one crucial boundary. A bit is the fundamental unit (0 or 1) and a byte is 8 bits. The prefixes AQA requires are the decimal SI prefixes — 1 kB = 1,000 bytes, not 1,024. The specification is explicit that kibi and mebi are not required, so multiply and divide by 1,000 throughout. Note the symbols too: lower-case b is a bit and capital B is a byte, a distinction that changes an answer by a factor of eight.

At a glance

A bit is the fundamental unit of information — either a 0 or a 1.
A byte is a group of 8 bits.
b = bit (lower case). B = byte (capital). Getting these the wrong way round is a factor-of-8 error.
1 kB = 1,000 bytes (kilo).
1 MB = 1,000 kB (mega).
1 GB = 1,000 MB (giga).
1 TB = 1,000 GB (tera).
These are the SI decimal prefixes, based on powers of 10 — not powers of 2.
Kibi, mebi and similar are NOT required — the specification says so explicitly.
To compare quantities, convert everything to the same unit first.

What you’ll learn

Mapped to the AQA GCSE 8525 syllabus (2027-2029).

Know that a bit is the fundamental unit of information and that a bit is either a 0 or a 1.
Know that a byte is a group of 8 bits, and that b represents bit while B represents byte.
Know that quantities of bytes can be described using prefixes, and know the names, symbols and corresponding values for the decimal prefixes kilo (1 kB is 1,000 bytes), mega (1 MB is 1,000 kilobytes), giga (1 GB is 1,000 megabytes) and tera (1 TB is 1,000 gigabytes).
Be able to compare quantities of bytes using these prefixes.

Bits and bytes

A bit is one binary digit. A byte is eight of them. Watch the symbols.

The bit

A bit is the fundamental unit of information. A bit is either a 0 or a 1.

That is the specification's own wording, and both halves matter — fundamental unit and either 0 or 1. The word "bit" is a contraction of binary digit.

A single bit can represent exactly two states: on/off, true/false, yes/no. It is the smallest piece of information a computer can store.

The byte

A byte is a group of 8 bits.

Eight bits give 2⁸ = 256 different combinations, representing the values 0 to 255 — which is exactly why the specification caps its conversion work at 255 (3.3.2), and why one byte can store a single ASCII character.

The symbols — a genuine trap

Symbol	Means	Note
b	bit	lower case
B	byte	capital

These differ by a factor of 8. A broadband connection advertised as "100 Mb/s" delivers 100 megabits per second, which is only 12.5 megabytes per second — internet providers quote bits because the number looks eight times better.

One byte holds one ASCII character, which is why 8 bits became the standard grouping.

Other groupings worth recognising

Name	Size
Bit	1 bit
Nibble	4 bits (half a byte, = 1 hex digit)
Byte	8 bits

The nibble is not named in 3.3.3 but appears in 3.3.2 as the group of four bits matching one hexadecimal digit.

AQA tip. In a calculation, always note whether the question is in bits or bytes. Converting between them means multiplying or dividing by 8, and mixing them up is the most costly error in file-size questions (3.3.6 and 3.3.7).

A bit is the fundamental unit of information — either a 0 or a 1.
A byte is a group of 8 bits, giving 256 combinations (0–255).
b = bit (lower case); B = byte (capital) — a factor-of-8 difference.
A nibble is 4 bits, matching one hexadecimal digit.
Converting bits ↔ bytes means multiplying or dividing by 8.

See the full worked example for units of information →

The decimal prefixes — powers of 1,000

kilo, mega, giga, tera. Each step is ×1,000. NOT 1,024.

The specification lists these four prefixes and their exact values:

Prefix	Symbol	Value	In bytes
kilo	kB	1 kB = 1,000 bytes	1,000
mega	MB	1 MB = 1,000 kB	1,000,000
giga	GB	1 GB = 1,000 MB	1,000,000,000
tera	TB	1 TB = 1,000 GB	1,000,000,000,000

Every step is a factor of 1,000. Moving up a prefix divides by 1,000; moving down multiplies by 1,000.

The 1,000 versus 1,024 question — read this carefully

The specification is unambiguous:

"The International System of Units (SI units) kilo, mega and so forth refer to values based on powers of 10. When referring to powers of 2 the terms kibi, mebi and so forth would normally be used but students do not need to know these."

So:

Use 1,000. 1 kB = 1,000 bytes.
Do not use 1,024 in your calculations.
Kibi (KiB), mebi (MiB) and similar are not examinable.

The specification does acknowledge that "historically the terms kilobyte, megabyte, etc have often been used to represent powers of 2" — which is why a computer sometimes reports a "1 TB" drive as roughly 931 GB. That history is context, not method. Every calculation you perform in this exam uses 1,000.

Moving between units

The bit-to-byte step is ÷8; every step above that is ÷1,000.

Worked conversions:

5,000 bytes → kB:     5,000 ÷ 1,000     = 5 kB
2.5 MB → kB:          2.5 × 1,000       = 2,500 kB
3 GB → MB:            3 × 1,000         = 3,000 MB
750,000 bytes → MB:   750,000 ÷ 1,000,000 = 0.75 MB

AQA tip. Write the conversion factor next to each step. Going up the ladder you divide; going down you multiply. Getting the direction wrong produces an answer a million times out, which a moment's sense-check will catch.

kilo (kB) = 1,000 bytes; mega (MB) = 1,000 kB; giga (GB) = 1,000 MB; tera (TB) = 1,000 GB.
These are SI decimal prefixes based on powers of 10.
Use 1,000 — NOT 1,024. Kibi and mebi are explicitly not required.
Going up the ladder divide by 1,000; going down multiply by 1,000.
The bit-to-byte step is a factor of 8, not 1,000.

See the full worked example for units of information →

Comparing quantities of bytes

Convert everything to the same unit first, then compare. Never compare across units.

The specification requires you to compare quantities of bytes using these prefixes.

The method

Choose one unit to work in — usually the smallest in the question, since that avoids decimals.
Convert every quantity into that unit.
Compare the numbers.

Worked comparison

Which is larger: 2,500 kB or 3 MB?

Convert both to kB (the smaller unit):

2,500 kB stays as 2,500 kB
3 MB = 3 × 1,000 = 3,000 kB

3 MB is larger.

Or convert both to MB:

2,500 kB = 2,500 ÷ 1,000 = 2.5 MB
3 MB stays as 3 MB ✓ same conclusion

Ordering a mixed list

Put these in ascending order: 0.5 GB, 800 MB, 2,000,000 bytes, 750 kB

Convert everything to bytes:

Quantity	In bytes
0.5 GB	0.5 × 1,000,000,000 = 500,000,000
800 MB	800 × 1,000,000 = 800,000,000
2,000,000 bytes	2,000,000
750 kB	750 × 1,000 = 750,000

Ascending order: 750 kB, 2,000,000 bytes, 0.5 GB, 800 MB

Notice that 0.5 GB is smaller than 800 MB — a comparison that is impossible to make reliably without converting, because the larger prefix makes 0.5 GB look bigger.

The bits-and-bytes trap in comparisons

Which is larger: 5,000 bits or 1 kB?

5,000 bits ÷ 8 = 625 bytes
1 kB = 1,000 bytes

1 kB is larger — even though 5,000 looks like the bigger number. Whenever bits appear alongside bytes, convert to bytes first.

AQA tip. Never try to compare quantities in different units by eye. Convert them all to a single unit, show that working, and then compare. Mark schemes credit the conversions, not just the conclusion.

To compare: convert everything to one unit first, then compare the numbers.
Working in the smallest unit avoids awkward decimals.
0.5 GB is smaller than 800 MB — the prefix alone is not a reliable guide.
If bits and bytes are mixed, convert to bytes first (÷8).
Show the conversions — mark schemes credit them.

See the full worked example for units of information →

Units in context — typical sizes

A rough sense of scale makes wrong answers obvious immediately.

Knowing approximate real-world sizes gives you a sanity check on any calculated file size in 3.3.6 and 3.3.7.

Item	Approximate size
One ASCII character	1 byte
A short text message	~100 bytes
A page of plain text	~2–3 kB
A small image	~100 kB
A high-quality photograph	~5 MB
A three-minute song	~30 MB (uncompressed)
A feature film	~1–5 GB
A modern laptop's storage	~500 GB – 2 TB

Why one byte per character?

One byte gives 2⁸ = 256 possible values — comfortably enough for the 128 characters of ASCII (3.3.5), which needs only 7 bits. This is why the byte became the standard grouping, and why a document of 2,000 characters occupies roughly 2,000 bytes (2 kB) in plain ASCII.

Using scale as a check

If a calculation gives a small photograph a size of 3 bytes or 40 TB, something has gone wrong — most likely a bits/bytes mix-up (a factor of 8) or a prefix step in the wrong direction (a factor of 1,000).

A worked sense-check. A 1,000 × 1,000 pixel image with 24-bit colour:

$1000 \times 1000 \times 24 = 24{,}000{,}000 \text{ bits}$ $24{,}000{,}000 \div 8 = 3{,}000{,}000 \text{ bytes} = 3 \text{ MB}$

3 MB for a megapixel photograph is entirely plausible, which confirms the arithmetic. Had the answer come out as 24 MB, the division by 8 had been forgotten.

AQA tip. After any file-size calculation, ask: "is this a believable size for this kind of file?" It costs two seconds and catches the two errors that cost the most marks in 3.3.6 and 3.3.7.

One ASCII character is one byte; a page of text is a few kB.
A photo is a few MB; a film is a few GB; a hard drive is hundreds of GB to a few TB.
One byte gives 256 values — ample for ASCII's 128 characters.
Use real-world scale to sense-check any calculated file size.
Implausible answers usually mean a factor-of-8 or factor-of-1,000 error.

See the full worked example for units of information →

Quick recap

A bit is the fundamental unit of information — either a 0 or a 1.
A byte is a group of 8 bits, giving 256 combinations (0–255).
b means bit (lower case) and B means byte (capital) — a factor-of-8 difference.
1 kB = 1,000 bytes; 1 MB = 1,000 kB; 1 GB = 1,000 MB; 1 TB = 1,000 GB.
These are SI decimal prefixes based on powers of 10 — use 1,000, not 1,024.
Kibi, mebi and similar terms are explicitly not required.
Going up the ladder divide by 1,000; going down multiply by 1,000.
To compare quantities, convert everything to the same unit first.
Use real-world scale as a sense-check on calculated file sizes.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

A bit is the fundamental unit of information — a 0 or a 1.
A byte is a group of 8 bits.
b = bit (lower case). B = byte (capital).
1 kB = 1,000 bytes. 1 MB = 1,000 kB. 1 GB = 1,000 MB. 1 TB = 1,000 GB.
Use 1,000 — NOT 1,024. Kibi/mebi are NOT required.
Bits → bytes: divide by 8. Bytes → bits: multiply by 8.
Up the ladder: ÷1,000. Down the ladder: ×1,000.

How it’s examined

3.3.3 is assessed on Paper 2 and is a reliable source of straightforward marks, but it is also where careless factor errors cost most. Three shapes recur. (1) Recall, 1–2 marks: state what a bit is, how many bits are in a byte, or the value of a named prefix (AO1) — pure definition marks. (2) Convert or compare, 2–4 marks: convert between bytes, kB, MB and GB, or place a mixed list of quantities in order, where the marks are for the conversions shown rather than only the conclusion (AO2). (3) Embedded in file-size questions, 3–6 marks: every bitmap calculation in 3.3.6 and sound calculation in 3.3.7 ends with converting bits to bytes and then to kB or MB, so this subtopic is examined indirectly far more often than directly (AO2). The two boundaries to bank are that AQA uses the decimal prefixes throughout — 1 kB = 1,000 bytes, and kibi/mebi are explicitly excluded — and that b and B differ by a factor of 8. Examiner reports repeatedly identify the same two losses: using 1,024 instead of 1,000, and forgetting to divide by 8 when a question asks for bytes.

Sources: AQA GCSE Computer Science 8525 specification (v1.3, 16 June 2025) — 3.3.3 Units of information; AQA GCSE Computer Science Paper 2 past papers, mark schemes and examiner reports. Last reviewed 2026-07-29.

Take this whole topic with you

Step-by-step worked examples — Units of information

Step-by-step solutions to past-paper-style questions on units of information, written exactly the way a tutor would explain them at the board.

Question type:

1Define a bit and a byte (3 marks)
Getting startedIdentify & classify• Paper 2 style — 3 marks (AO1)• bit, byte, define
Question
(a) State what a bit is. (b) State how many bits are in a byte. (c) State what the symbols b and B represent. (3 marks)
Step-by-step solution
1. Step 1
  (a) Use the specification's wording: a bit is the fundamental unit of information, and it is either a 0 or a 1.
2. Step 2
  (b) A byte is a group of 8 bits.
3. Step 3
  (c) Lower-case b is a bit; capital B is a byte. The two differ by a factor of eight.
Answer
(a) A bit is the fundamental unit of information, and is either a 0 or a 1 (1). (b) A byte is a group of 8 bits (1). (c) b represents a bit and B represents a byte (1). (3)
Examiner tip
Part (a) should include both halves of the specification's wording — 'fundamental unit of information' and 'either 0 or 1'. Part (c) matters in practice: a connection advertised at 100 Mb/s delivers 100 megabits per second, which is only 12.5 MB/s.
2Convert between units using the decimal prefixes (4 marks)
Getting startedDirect calculation• Paper 2 style — 4 marks (AO2)• prefix, conversion, kilobyte
Question
Convert each of the following, showing your working.

(a) 4,000 bytes into kilobytes (b) 2.5 MB into kilobytes (c) 6 GB into megabytes (d) 3,000,000 bytes into megabytes (4 marks)
Step-by-step solution
1. Step 1
  (a) Going UP the ladder, so divide. 4,000 ÷ 1,000 = 4 kB.
  $4{,}000 \div 1{,}000 = 4$
2. Step 2
  (b) Going DOWN the ladder, so multiply. 2.5 × 1,000 = 2,500 kB.
  $2.5 \times 1{,}000 = 2{,}500$
3. Step 3
  (c) Down one step. 6 × 1,000 = 6,000 MB.
4. Step 4
  (d) Up two steps, so divide by 1,000 twice — that is, divide by 1,000,000. 3,000,000 ÷ 1,000,000 = 3 MB.
  $3{,}000{,}000 \div 1{,}000{,}000 = 3$
Answer
(a) 4,000 ÷ 1,000 = 4 kB (1) (b) 2.5 × 1,000 = 2,500 kB (1) (c) 6 × 1,000 = 6,000 MB (1) (d) 3,000,000 ÷ 1,000,000 = 3 MB (1) (4)
Examiner tip
Every step uses 1,000, not 1,024 — the specification uses the SI decimal prefixes and states that kibi and mebi are not required. The direction is the thing to fix: going up the ladder you divide, going down you multiply. Part (d) crosses two steps, hence dividing by a million.
3Convert between bits and bytes (3 marks)
Getting startedDirect calculation• Paper 2 style — 3 marks (AO2)• bits, bytes, conversion
Question
(a) Convert 4,000 bits into bytes. (b) Convert 250 bytes into bits. (c) A file is 16,000 bits. State its size in kilobytes. (3 marks)
Step-by-step solution
1. Step 1
  (a) Bits → bytes: divide by 8. 4,000 ÷ 8 = 500 bytes.
  $4{,}000 \div 8 = 500$
2. Step 2
  (b) Bytes → bits: multiply by 8. 250 × 8 = 2,000 bits.
3. Step 3
  (c) Two steps. First bits → bytes: 16,000 ÷ 8 = 2,000 bytes. Then bytes → kB: 2,000 ÷ 1,000 = 2 kB.
  $16{,}000 \div 8 = 2{,}000 \div 1{,}000 = 2$
Answer
(a) 4,000 ÷ 8 = 500 bytes (1) (b) 250 × 8 = 2,000 bits (1) (c) 16,000 ÷ 8 = 2,000 bytes, then 2,000 ÷ 1,000 = 2 kB (1) (3)
Examiner tip
The bit-to-byte step is a factor of 8, whereas every step above it is a factor of 1,000 — mixing the two up is the most costly error in this area. Part (c) is the standard two-stage conversion that ends every file-size calculation in 3.3.6 and 3.3.7.
4Compare and order quantities of bytes (5 marks)
Building confidenceMulti-step problem• Paper 2 style — 5 marks (AO2)• compare, order, prefix
Question
Place the following quantities in ascending order (smallest first), showing your working.

0.5 GB · 800 MB · 2,000,000 bytes · 750 kB (5 marks)
Step-by-step solution
1. Step 1
  Choose one unit for the comparison. Bytes is the safest here, since it avoids decimals entirely.
2. Step 2
  0.5 GB = 0.5 × 1,000,000,000 = 500,000,000 bytes.
3. Step 3
  800 MB = 800 × 1,000,000 = 800,000,000 bytes.
4. Step 4
  750 kB = 750 × 1,000 = 750,000 bytes, and 2,000,000 bytes needs no conversion.
5. Step 5
  Order the byte values. Note that 0.5 GB turns out to be smaller than 800 MB, which is impossible to judge by eye.
Answer
Converting everything to bytes:

Quantity In bytes
750 kB 750 × 1,000 = 750,000
2,000,000 bytes 2,000,000
0.5 GB 0.5 × 1,000,000,000 = 500,000,000
800 MB 800 × 1,000,000 = 800,000,000

(3 marks for correct conversions.)

Ascending order: 750 kB, 2,000,000 bytes, 0.5 GB, 800 MB (2 marks)
Examiner tip
The deliberate trap is that 0.5 GB is smaller than 800 MB — the larger prefix makes it look bigger, and no reliable comparison is possible without converting. Marks are awarded for the conversions, not just the final ordering, so the working must be shown.
5Apply the decimal prefix rule correctly (4 marks)
Building confidenceWord problem• Paper 2 style — 4 marks (AO1/AO2)• prefix, 1000, SI
Question
A student calculates that 3 MB is 3,145,728 bytes, using 1,024 for each conversion step.

(a) State the value AQA requires for each prefix step. (b) Give the correct answer. (c) Explain why the student's figure is not wrong in every context, but is wrong for this specification. (4 marks)
Step-by-step solution
1. Step 1
  (a) The specification uses the SI decimal prefixes, so each step is a factor of 1,000.
2. Step 2
  (b) 3 MB = 3 × 1,000 × 1,000 = 3,000,000 bytes.
  $3 \times 1{,}000 \times 1{,}000 = 3{,}000{,}000$
3. Step 3
  (c) Acknowledge the history. The specification notes that the terms kilobyte and megabyte have often been used historically to mean powers of 2, which is where 1,024 comes from.
4. Step 4
  But state the rule. For powers of 2 the correct terms are kibi and mebi, and AQA states explicitly that students do not need to know these — so all examinable calculations use 1,000.
Answer
(a) Each prefix step is a factor of 1,000 — these are the SI decimal prefixes, based on powers of 10 (1).

(b) 3 MB = 3 × 1,000 × 1,000 = 3,000,000 bytes (1).

(c) The student's figure comes from using 1,024 (2¹⁰) for each step, and the specification acknowledges that terms such as kilobyte and megabyte have historically been used to mean powers of 2, so the value is not meaningless (1). However, when referring to powers of 2 the correct terms are kibi and mebi, which the specification states students do not need to know — so for this specification all calculations must use 1,000 (1). (4)
Examiner tip
This is a genuine real-world ambiguity that the specification resolves explicitly, which is why it is worth knowing the reasoning as well as the rule. The practical takeaway is simple: always use 1,000 in the exam, and never introduce 1,024 into a calculation.
6Use scale to sense-check a calculated size (4 marks)
Building confidenceWord problem• Paper 2 style — 4 marks (AO3)• sense-check, file-size, estimation
Question
A student calculates that a 1,000 × 1,000 pixel photograph with a colour depth of 24 bits has a file size of 24 MB.

(a) Show the correct calculation. (b) Identify the student's error. (c) Explain how a rough sense of typical file sizes would have revealed the mistake. (4 marks)
Step-by-step solution
1. Step 1
  (a) Size in bits = width × height × colour depth = 1,000 × 1,000 × 24 = 24,000,000 bits.
  $1000 \times 1000 \times 24 = 24{,}000{,}000 \text{ bits}$
2. Step 2
  Convert to bytes by dividing by 8: 24,000,000 ÷ 8 = 3,000,000 bytes.
  $24{,}000{,}000 \div 8 = 3{,}000{,}000$
3. Step 3
  Convert to MB: 3,000,000 ÷ 1,000,000 = 3 MB.
4. Step 4
  (b) The student has forgotten to divide by 8, quoting the size in bits while labelling it MB — an error of exactly a factor of eight.
5. Step 5
  (c) A one-megapixel photograph is typically a few MB, so 3 MB is plausible while 24 MB is roughly eight times too large — a discrepancy that immediately signals a bits/bytes error.
Answer
(a) Size in bits = 1,000 × 1,000 × 24 = 24,000,000 bits (1) Size in bytes = 24,000,000 ÷ 8 = 3,000,000 bytes Size in MB = 3,000,000 ÷ 1,000,000 = 3 MB (1)

(b) The student forgot to divide by 8, so they have quoted the size in bits but labelled it as megabytes — an error of exactly a factor of eight (1).

(c) A one-megapixel photograph would typically be a few megabytes, so 3 MB is entirely plausible whereas 24 MB is around eight times larger than expected (1). Recognising that mismatch immediately points to a missing division by 8, which is the most common error in file-size calculations.
Examiner tip
The sense-check habit is worth building because it catches the two dominant errors in 3.3.6 and 3.3.7 — a missing ÷8 (factor of eight) and a prefix step in the wrong direction (factor of 1,000). Both produce answers that are wildly implausible for the kind of file described.
7Compare quantities mixing bits and bytes (6 marks)
StretchMulti-step problem• Paper 2 style — 6 marks (AO2/AO3)• bits, bytes, compare, network
Question
A student must download a 12 MB file over a connection advertised as 8 Mb/s.

(a) Explain what Mb/s means and how it differs from MB/s. (b) Calculate how long the download should take. (c) Explain why internet providers advertise speeds in megabits rather than megabytes. (6 marks)
Step-by-step solution
1. Step 1
  (a) Lower-case b is a bit, so Mb/s means megabits per second, whereas MB/s would mean megabytes per second — eight times larger.
2. Step 2
  (b) Convert the speed to the same unit as the file. 8 Mb/s ÷ 8 = 1 MB/s.
  $8 \text{ Mb/s} \div 8 = 1 \text{ MB/s}$
3. Step 3
  Then divide the file size by the speed. 12 MB ÷ 1 MB/s = 12 seconds.
4. Step 4
  Alternative route: convert the file to bits instead — 12 MB × 8 = 96 Mb, then 96 ÷ 8 = 12 seconds. Same answer, confirming the working.
5. Step 5
  (c) Quoting bits makes the number eight times larger, so the connection sounds considerably faster in marketing material.
Answer
(a) Mb/s means megabits per second, because the lower-case b stands for bit (1). MB/s would mean megabytes per second, and since one byte is 8 bits, MB/s is eight times larger than Mb/s (1).

(b) Convert the connection speed into megabytes per second: 8 Mb/s ÷ 8 = 1 MB/s (1)

Time = file size ÷ speed = 12 MB ÷ 1 MB/s = 12 seconds (1)

(Checking the other way: 12 MB × 8 = 96 Mb, and 96 Mb ÷ 8 Mb/s = 12 seconds ✓)

(c) Because a quantity expressed in bits is eight times larger than the same quantity in bytes, advertising a connection as '8 Mb/s' rather than '1 MB/s' makes it sound considerably faster (1) — the figure is technically accurate but presents the service in the most favourable light, which is why download times often feel slower than users expect (1).
Examiner tip
This is the most practically useful application of the b/B distinction and appears in scenario questions. Note the answer works in either direction — converting the speed to MB/s or the file to Mb — and doing both is a strong self-check. The marketing point in part (c) is a legitimate AO3 observation.
8Work through a multi-stage unit calculation (6 marks)
StretchMulti-step problem• Paper 2 style — 6 marks (AO2)• multi-step, storage, capacity
Question
A school stores exam papers as scanned images. Each scan is 1.5 MB and there are 40,000 scans.

(a) Calculate the total storage required in GB. (b) The school has a 500 GB drive that is already 60% full. Determine whether the scans will fit, showing your reasoning. (c) State the total in bits, giving your answer in standard form or as a full number. (6 marks)
Step-by-step solution
1. Step 1
  (a) Total in MB = 1.5 × 40,000 = 60,000 MB.
  $1.5 \times 40{,}000 = 60{,}000 \text{ MB}$
2. Step 2
  Convert MB → GB by dividing by 1,000: 60,000 ÷ 1,000 = 60 GB.
3. Step 3
  (b) Find the free space. The drive is 60% full, so 40% is free: 500 × 0.4 = 200 GB available.
  $500 \times 0.4 = 200 \text{ GB free}$
4. Step 4
  Compare: 60 GB needed against 200 GB free — so yes, the scans will fit, with 140 GB to spare.
5. Step 5
  (c) GB → bytes: 60 × 1,000,000,000 = 60,000,000,000 bytes.
6. Step 6
  Bytes → bits: multiply by 8. 60,000,000,000 × 8 = 480,000,000,000 bits.
  $60{,}000{,}000{,}000 \times 8 = 4.8 \times 10^{11}$
Answer
(a) Total = 1.5 MB × 40,000 = 60,000 MB (1) 60,000 ÷ 1,000 = 60 GB (1)

(b) The drive is 60% full, so the free space is 40% of 500 GB: 500 × 0.4 = 200 GB free (1)

Since 60 GB < 200 GB, the scans will fit, leaving 140 GB spare (1).

(c) 60 GB → bytes: 60 × 1,000,000,000 = 60,000,000,000 bytes (1) bytes → bits: 60,000,000,000 × 8 = 480,000,000,000 bits (or 4.8 × 10¹¹) (1)
Examiner tip
Part (b) adds a percentage step to the unit conversion, which is typical of applied questions — the key move is finding the free space rather than the used space. Part (c) exercises the full chain GB → bytes → bits, using ÷1,000 twice and then ×8, and a sense-check confirms the enormous figure is expected when quoting a large capacity in bits.
9Explain why a byte is 8 bits (6 marks)
StretchWord problem• Paper 2 style — 6 marks (AO3)• byte, explain, ascii, 256
Question
(a) Calculate how many different values can be stored in one byte, showing your reasoning. (b) Explain why this makes a byte a convenient size for storing a single character. (c) Explain why quantities of storage are described using prefixes rather than being written out in full. (6 marks)
Step-by-step solution
1. Step 1
  (a) Each bit independently holds 0 or 1, so 8 bits give 2⁸ combinations.
  $2^8 = 256$
2. Step 2
  State the range. Those 256 patterns represent the values 0 to 255, since the count includes zero.
3. Step 3
  (b) Link to ASCII. 7-bit ASCII defines 128 characters, so 256 values is more than enough to store any single character with room to spare.
4. Step 4
  Draw the consequence. One character therefore occupies exactly one byte, which is why a plain-text document of 2,000 characters is about 2 kB.
5. Step 5
  (c) Reason 1 — readability. Writing 60,000,000,000 bytes is unwieldy and easy to miscount; '60 GB' is immediately clear.
6. Step 6
  Reason 2 — comparison and error reduction. Consistent prefixes make quantities easy to compare and reduce the chance of miscounting zeros.
Answer
(a) Each of the 8 bits can independently be 0 or 1, so the number of combinations is 2⁸ = 256 (1). These represent the values 0 to 255, because the count includes zero (1).

(b) 7-bit ASCII defines 128 characters, so a byte's 256 possible values is more than enough to store any single character with capacity to spare (1). This means one character occupies exactly one byte, which makes text storage simple to calculate — a plain-text document of 2,000 characters occupies roughly 2,000 bytes, or 2 kB (1).

(c) Storage quantities are frequently enormous, and writing them in full — such as 60,000,000,000 bytes — is unwieldy, hard to read and very easy to miscount by a zero (1). Prefixes express the same quantity compactly as 60 GB, which is clearer, quicker to write and makes quantities far easier to compare at a glance (1).
Examiner tip
Part (b) is the historically decisive reason the byte became standard, and linking 256 to ASCII's 128 characters connects 3.3.3 to 3.3.5. Note the recurring counting point from 3.3.1: 256 values, maximum 255, because zero occupies one of the patterns.

Quantity	In bytes
750 kB	750 × 1,000 = 750,000
2,000,000 bytes	2,000,000
0.5 GB	0.5 × 1,000,000,000 = 500,000,000
800 MB	800 × 1,000,000 = 800,000,000

Model Answers — Units of information

High-scoring sample answers for units of information on the AQA GCSE 8525 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
Paper 2 style — AO11 mark
State what is meant by a bit. (1 mark)
Model answer
A bit is the fundamental unit of information, which is either a 0 or a 1 (1).
Why this scores
Both halves of the specification's wording are worth giving — 'fundamental unit of information' and 'either 0 or 1'. The word 'bit' is a contraction of binary digit.
Question 2
Paper 2 style — AO12 marks
State the number of bytes in 1 kB and the number of kilobytes in 1 MB. (2 marks)
Model answer
1 kB = 1,000 bytes (1). 1 MB = 1,000 kB (1).
Why this scores
The specification uses the SI decimal prefixes, so the answer is 1,000 and never 1,024. It states explicitly that when referring to powers of 2 the terms kibi and mebi would be used, and that students do not need to know these.
Question 3
Paper 2 style — AO1/AO22 marks
A broadband connection is advertised as 50 Mb/s. State what the lower-case b means and calculate the equivalent speed in MB/s. (2 marks)
Model answer
The lower-case b means bit, so the connection transfers 50 megabits per second (1).

Since 1 byte = 8 bits: 50 ÷ 8 = 6.25 MB/s (1).
Why this scores
This distinction matters practically as well as in the exam — providers quote bits because the figure is eight times larger and therefore sounds faster. Capital B would mean bytes.
Question 4
Paper 2 style — AO23 marks
A file is 24,000 bits in size. Express this in kilobytes, showing each step. (3 marks)
Model answer
Step 1 — bits to bytes (divide by 8): 24,000 ÷ 8 = 3,000 bytes (1)

Step 2 — bytes to kilobytes (divide by 1,000): 3,000 ÷ 1,000 = 3 kB (1)

Answer: 3 kB (1)
Why this scores
Note the two different factors: the bit-to-byte step is ÷8 while every prefix step is ÷1,000. This exact two-stage chain is how every bitmap and sound file-size calculation in 3.3.6 and 3.3.7 finishes.
Question 5
Paper 2 style — AO23 marks
State which is larger, 6,000 kB or 5 MB, showing your working. (3 marks)
Model answer
Convert both to the same unit. Working in kilobytes:

6,000 kB remains 6,000 kB (1)

5 MB = 5 × 1,000 = 5,000 kB (1)

Since 6,000 > 5,000, 6,000 kB is larger (1).

(Checking in MB: 6,000 kB ÷ 1,000 = 6 MB, which is indeed greater than 5 MB ✓)
Why this scores
The marks are for the conversions, not just the conclusion, so both quantities must be shown in a common unit. A quantity with the larger prefix is not automatically bigger — 0.5 GB is smaller than 800 MB, for instance.
Question 6
Paper 2 style — AO23 marks
A memory card has a capacity of 32 GB. Each photograph taken by a camera is 4 MB. Calculate how many photographs the card can hold. (3 marks)
Model answer
Convert the card capacity into the same unit as the photographs:

32 GB = 32 × 1,000 = 32,000 MB (1)

Number of photographs = 32,000 ÷ 4 (1) = 8,000 photographs (1)
Why this scores
Converting to the smaller unit keeps the arithmetic in whole numbers and avoids decimals. A quick sense-check confirms the result: thousands of photographs on a 32 GB card is entirely plausible, whereas an answer of 8 or 8,000,000 would signal a factor error.
Question 7
Paper 2 style — AO2/AO36 marks
A video streaming service transmits at 20 Mb/s.

(a) Calculate how many megabytes are transferred in one minute. (b) A user has a monthly data allowance of 100 GB. Calculate how many hours of streaming this allows. (c) Explain one reason why the actual figure may be lower than your calculation. (6 marks)
Model answer
(a) Convert the rate to megabytes per second: 20 Mb/s ÷ 8 = 2.5 MB/s (1)

In one minute: 2.5 × 60 = 150 MB per minute (1)

(b) Convert the allowance to MB: 100 GB = 100 × 1,000 = 100,000 MB (1)

Minutes of streaming = 100,000 ÷ 150 = 666.67 minutes (1)

In hours: 666.67 ÷ 60 ≈ 11.1 hours (1)

(c) The actual figure is likely to be lower because the calculation assumes the connection sustains its maximum advertised rate continuously, whereas real speeds fluctuate with network congestion; additional data is also consumed by overheads such as protocol headers and error-checking information, which count towards the allowance without being part of the video itself (1).
Why this scores
A three-stage conversion chain — bits to bytes, seconds to minutes, GB to MB — where each step must use the correct factor (÷8 for bits, ×60 for time, ×1,000 for the prefix). Part (c) rewards realism, and either explanation (fluctuating speed or protocol overhead) is creditworthy.
Question 8
Paper 2 style — AO1/AO36 marks
Explain why AQA specifies that 1 kB is 1,000 bytes, and discuss why some computer systems appear to report different figures. (6 marks)
Model answer
Why AQA uses 1,000. The prefixes kilo, mega, giga and tera are SI units, which are defined as powers of 10 — kilo means one thousand in every scientific context, from kilometres to kilograms (1). Using the same definitions for bytes keeps the units consistent with the rest of science and makes them unambiguous, so the specification states that 1 kB is 1,000 bytes and every examinable calculation uses that value (1).

Why some systems report differently. Computer memory is addressed in powers of 2, so 2¹⁰ = 1,024 is a natural grouping for hardware, and because 1,024 is close to 1,000 early computing borrowed the prefix 'kilo' to describe it (1). This means the same physical quantity can be expressed two different ways: a drive sold as 1 TB contains 10¹² bytes using SI units, but an operating system dividing by 1,024 repeatedly reports it as approximately 931 GB (1).

How the ambiguity is resolved. The correct terms for powers of 2 are kibi (KiB) and mebi (MiB), which remove the confusion entirely (1). However, the specification states explicitly that students do not need to know these terms, so for this course every calculation uses 1,000 and 1,024 should never appear (1).
Why this scores
This question rewards understanding the ambiguity rather than merely following the rule. The essential exam point is unambiguous — always use 1,000 — but explaining where 1,024 comes from, and that kibi/mebi exist to resolve it, demonstrates genuine command of the specification's stated boundary.
Question 9
Paper 2 style — AO2/AO36 marks
A plain-text book contains approximately 400,000 characters, stored using 7-bit ASCII but with each character occupying one byte.

(a) Calculate the file size in kB. (b) Calculate how many such books could be stored on a 500 GB drive. (c) Explain why storing each 7-bit character in a full byte wastes space, and state approximately how much. (6 marks)
Model answer
(a) One character occupies one byte, so: 400,000 characters = 400,000 bytes (1) 400,000 ÷ 1,000 = 400 kB (1)

(b) Convert the drive capacity to kB: 500 GB = 500 × 1,000 × 1,000 = 500,000,000 kB (1)

Number of books = 500,000,000 ÷ 400 = 1,250,000 books (1)

(c) 7-bit ASCII needs only 7 bits per character, but each character is stored in a full 8-bit byte, so one bit in every eight is unused (1). That is a waste of 1/8, or 12.5%, of the storage — for this book, 400,000 unused bits, which is 50,000 bytes or 50 kB (1).
Why this scores
Part (c) makes the link between 3.3.3 and 3.3.5 explicit: ASCII is defined as 7-bit while storage is organised in 8-bit bytes, so the eighth bit is spare. Historically that bit was used for error checking or to extend the character set. The wasted fraction of exactly 1/8 = 12.5% is the figure to quote.

Key Formulae — Units of information

The formulae you need to memorise for units of information on the AQA GCSE 8525 paper, with every variable defined in plain English and a note on when to use it.

Bits and bytes
$1 \text{ byte} = 8 \text{ bits}$
$bits → bytes$
Divide by 8
$bytes → bits$
Multiply by 8
When to use
Whenever a question mixes bits and bytes, and at the end of every file-size calculation in 3.3.6 and 3.3.7, which produce a result in bits.
Example
24,000,000 bits ÷ 8 = 3,000,000 bytes
Decimal (SI) prefixes
$1\,\text{kB} = 10^3 \text{ B},\;\; 1\,\text{MB} = 10^3 \text{ kB},\;\; 1\,\text{GB} = 10^3 \text{ MB},\;\; 1\,\text{TB} = 10^3 \text{ GB}$
$Up the ladder$
Divide by 1,000
$Down the ladder$
Multiply by 1,000
When to use
Every conversion between units of storage. AQA uses the SI decimal prefixes — always 1,000, never 1,024. Kibi and mebi are explicitly not required.
Example
60,000 MB ÷ 1,000 = 60 GB
Number of values in n bits
$\text{values} = 2^n$
$n$
The number of bits
$Range$
0 to 2ⁿ − 1, because the count includes zero
When to use
Calculating how many different values a given number of bits can store — 8 bits give 256 values (0–255), 4 bits give 16 (0–15).
Example
2⁸ = 256 values, representing 0 to 255

Key Definitions and Keywords — Units of information

Definitions to memorise and the exact keywords mark schemes credit for units of information answers — sharpened from recent examiner reports for the 2026 AQA GCSE 8525 sitting.

Bit
Examiner keyword
The fundamental unit of information, which is either a 0 or a 1. The word is a contraction of binary digit.
Related: Byte, Nibble
Byte
Examiner keyword
A group of 8 bits. One byte can store 2⁸ = 256 different values, representing 0 to 255, which is enough for any single ASCII character.
Related: Bit, Kilobyte
b and B
Examiner keyword
Lower-case b represents a bit; capital B represents a byte. The two differ by a factor of 8, so confusing them is a significant error.
Example
100 Mb/s is 100 megabits per second, equal to 12.5 MB/s.
Related: Bit, Byte
Kilobyte (kB)
Examiner keyword
1,000 bytes. AQA uses the SI decimal prefix based on powers of 10, not 1,024.
Related: Megabyte, SI prefix
Megabyte (MB)
Examiner keyword
1,000 kilobytes, which is 1,000,000 bytes.
Related: Kilobyte, Gigabyte
Gigabyte (GB)
Examiner keyword
1,000 megabytes, which is 1,000,000,000 bytes.
Related: Megabyte, Terabyte
Terabyte (TB)
Examiner keyword
1,000 gigabytes, which is 1,000,000,000,000 bytes.
Related: Gigabyte
SI (decimal) prefix
Examiner keyword
A prefix based on powers of 10, as used throughout science. Kilo, mega, giga and tera each represent a factor of 1,000, and these are the values AQA requires.
Related: Kilobyte, Binary prefix
Binary prefix (kibi, mebi)
Examiner keyword
Prefixes based on powers of 2, where 1 KiB = 1,024 bytes. The specification states explicitly that students do NOT need to know these terms.
Related: SI (decimal) prefix
Nibble
A group of 4 bits — half a byte. Corresponds to exactly one hexadecimal digit, which is why hexadecimal is used as shorthand for binary.
Example
1101 = D
Related: Bit, Byte, Hexadecimal
File size
Examiner keyword
The amount of storage a file occupies, normally quoted in bytes with an appropriate prefix. Calculations in 3.3.6 and 3.3.7 produce a result in bits, which must then be divided by 8.
Related: Byte, Kilobyte
Storage capacity
The total amount of data a storage device can hold, normally expressed in GB or TB. Comparing capacity with file sizes requires converting both to a common unit.
Related: Gigabyte, Terabyte

Common Mistakes and Misconceptions — Units of information

The traps other students keep falling into on units of information questions — taken from recent AQA GCSE 8525 examiner reports and mark schemes — and how to avoid them.

✕Using 1,024 instead of 1,000 when converting between prefixes.
AQA 8525 specification 3.3.3 additional information states the SI prefixes refer to powers of 10 and that kibi/mebi are not required.
Why it happens
1,024 appears constantly in computing contexts and in what operating systems report, so it feels like the 'proper' computing value — and many textbooks still teach it.
How to avoid it
AQA uses the SI decimal prefixes: 1 kB = 1,000 bytes, and every step up the ladder is another factor of 1,000. The specification states that the powers-of-2 terms are kibi and mebi and that students do not need to know them, so 1,024 should never appear in an answer.
✕Confusing bits with bytes, or reading Mb as megabytes rather than megabits.
AQA 8525 specification 3.3.3 states explicitly that b represents bit and B represents byte.
Why it happens
The symbols differ only in capitalisation, which is easy to overlook when scanning a question quickly — and the words themselves sound almost identical when read.
How to avoid it
Fix the symbols: lower-case b is a bit, capital B is a byte, and they differ by a factor of 8. Before starting any calculation, underline whether the question is in bits or bytes. A 100 Mb/s connection delivers 12.5 MB/s.
✕Forgetting to divide by 8 when a file-size calculation gives a result in bits but the question asks for bytes.
AQA 8525 specification 3.3.6 gives both forms of the bitmap formula — in bits and in bytes — precisely because the conversion is required.
Why it happens
The main calculation feels finished once the multiplication is done, and the unit conversion reads like an afterthought rather than part of the answer.
How to avoid it
The bitmap and sound formulae both produce bits. Always finish by converting: ÷8 for bytes, then ÷1,000 for kB. Sense-check the result — a photograph of 24 MB when you expected a few MB is exactly a factor of eight out.
✕Multiplying when you should divide, or vice versa, when moving between prefixes.
Direction errors produce answers a million times out and are readily caught by a quick plausibility check.
Why it happens
Both operations use the same factor of 1,000, so the arithmetic feels symmetrical and the direction is easy to reverse under time pressure.
How to avoid it
Going up the ladder to a larger unit, the number gets smaller, so you divide. Going down to a smaller unit, the number gets larger, so you multiply. Sense-check: 5,000 bytes is 5 kB, not 5,000,000 kB.
✕Comparing quantities in different units by eye, without converting them first.
AQA 8525 specification 3.3.3 requires students to be able to compare quantities of bytes using the prefixes.
Why it happens
A larger prefix intuitively suggests a larger quantity, so 0.5 GB looks bigger than 800 MB and the comparison seems obvious enough to skip.
How to avoid it
Always convert everything to one unit before comparing — usually the smallest, to avoid decimals. 0.5 GB is 500 MB, which is genuinely smaller than 800 MB. Mark schemes credit the conversions, not just the conclusion.
✕Saying a byte can store values up to 256, or that it holds 255 different values.
This mirrors the counting distinction in 3.3.1 and 3.3.2, where the specification's stated maximum is 255.
Why it happens
The two numbers are both firmly associated with 'one byte' and differ by only one, so under pressure the wrong one is recalled.
How to avoid it
A byte holds 256 different values, running from 0 to 255 — the count includes zero, so the maximum value is one less than the count. The same reasoning gives 4 bits sixteen values from 0 to 15.

Units of Information — AQA GCSE Computer Science 8525 (3.3.3)

Symbol

Means

Note

bit

lower case

byte

capital

Name

Size

Bit

1 bit

Nibble

4 bits (half a byte, = 1 hex digit)

Byte

8 bits

Prefix

Symbol

Value

In bytes

kilo

1 kB = 1,000 bytes

1,000

mega

1 MB = 1,000 kB

1,000,000

giga

1 GB = 1,000 MB

1,000,000,000

tera

1 TB = 1,000 GB

1,000,000,000,000

5,000 bytes → kB: 5,000 ÷ 1,000 = 5 kB 2.5 MB → kB: 2.5 × 1,000 = 2,500 kB 3 GB → MB: 3 × 1,000 = 3,000 MB 750,000 bytes → MB: 750,000 ÷ 1,000,000 = 0.75 MB

Quantity

In bytes

0.5 GB

0.5 × 1,000,000,000 = 500,000,000

800 MB

800 × 1,000,000 = 800,000,000

2,000,000 bytes

2,000,000

750 kB

750 × 1,000 = 750,000

Item

Approximate size

One ASCII character

1 byte

A short text message

~100 bytes

A page of plain text

~2–3 kB

A small image

~100 kB

A high-quality photograph

~5 MB

A three-minute song

~30 MB (uncompressed)

A feature film

~1–5 GB

A modern laptop's storage

~500 GB – 2 TB

Quantity	In bytes
750 kB	750 × 1,000 = 750,000
2,000,000 bytes	2,000,000
0.5 GB	0.5 × 1,000,000,000 = 500,000,000
800 MB	800 × 1,000,000 = 800,000,000

Quantity

In bytes

750 kB

750 × 1,000 = 750,000

2,000,000 bytes

2,000,000

0.5 GB

0.5 × 1,000,000,000 = 500,000,000

800 MB

800 × 1,000,000 = 800,000,000

Units of information

Detailed Study Notes

At a glance

What you’ll learn

Bits and bytes

The bit

The byte

The symbols — a genuine trap

Other groupings worth recognising

The decimal prefixes — powers of 1,000

The 1,000 versus 1,024 question — read this carefully

Moving between units

Comparing quantities of bytes

The method

Worked comparison

Ordering a mixed list

The bits-and-bytes trap in comparisons

Units in context — typical sizes

Why one byte per character?

Using scale as a check

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Define a bit and a byte (3 marks)

2Convert between units using the decimal prefixes (4 marks)

3Convert between bits and bytes (3 marks)

4Compare and order quantities of bytes (5 marks)

5Apply the decimal prefix rule correctly (4 marks)

6Use scale to sense-check a calculated size (4 marks)

7Compare quantities mixing bits and bytes (6 marks)

8Work through a multi-stage unit calculation (6 marks)

9Explain why a byte is 8 bits (6 marks)

Bits and bytes

Decimal (SI) prefixes

Number of values in n bits

Bit

Byte

b and B

Kilobyte (kB)

Megabyte (MB)

Gigabyte (GB)

Terabyte (TB)

SI (decimal) prefix

Binary prefix (kibi, mebi)

Nibble

File size

Storage capacity

✕Using 1,024 instead of 1,000 when converting between prefixes.

✕Confusing bits with bytes, or reading Mb as megabytes rather than megabits.

✕Forgetting to divide by 8 when a file-size calculation gives a result in bits but the question asks for bytes.

✕Multiplying when you should divide, or vice versa, when moving between prefixes.

✕Comparing quantities in different units by eye, without converting them first.

✕Saying a byte can store values up to 256, or that it holds 255 different values.

Units of information

Detailed Study Notes

At a glance

What you’ll learn

Bits and bytes

The bit

The byte

The symbols — a genuine trap

Other groupings worth recognising

The decimal prefixes — powers of 1,000

The 1,000 versus 1,024 question — read this carefully

Moving between units

Comparing quantities of bytes

The method

Worked comparison

Ordering a mixed list

The bits-and-bytes trap in comparisons

Units in context — typical sizes

Why one byte per character?

Using scale as a check

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1Define a bit and a byte (3 marks)

✕Confusing bits with bytes, or reading `Mb` as megabytes rather than megabits.