Units of Measurement

Length tells you how long, tall or far something is. The metric units, from smallest to largest, are the millimetre (mm), centimetre (cm), metre (m) and kilometre (km).

The clever part is that they are linked by powers of 10:

So the key facts are: $10\text{ mm} = 1\text{ cm}$, $100\text{ cm} = 1\text{ m}$ and $1000\text{ m} = 1\text{ km}$.

Choosing a sensible unit matters too. You would measure a pencil in centimetres, your height in metres, and the distance to another town in kilometres. Picking the right unit keeps the numbers easy to handle.

Mass tells you how heavy something is. The metric units are the gram (g), kilogram (kg) and tonne (t):

• $1000\text{ g} = 1\text{ kg}$
• $1000\text{ kg} = 1\text{ tonne}$

A paperclip is about 1 gram, a bag of sugar is 1 kilogram, and a small car is roughly 1 tonne.

Capacity tells you how much liquid a container can hold. The metric units are the millilitre (ml) and litre (l):

• $1000\text{ ml} = 1\text{ litre}$

Just like length, these units jump by powers of 10, so converting is the same idea: multiply to reach a smaller unit, divide to reach a bigger one. For example, $2.5\text{ kg} = 2.5 \times 1000 = 2500\text{ g}$, and $3000\text{ ml} = 3000 \div 1000 = 3\text{ litres}$.

Converting units is a skill you will use again and again, so here is a reliable method.

Step 1 — Find the link. Recall the conversion fact, such as $1\text{ m} = 100\text{ cm}$.

Step 2 — Decide the direction.

• Going to a smaller unit (m → cm): the number gets bigger, so multiply.
• Going to a bigger unit (cm → m): the number gets smaller, so divide.

Step 3 — Carry out the calculation.

• $4.2\text{ km}$ into metres: km → m is smaller, so multiply. $4.2 \times 1000 = 4200\text{ m}$.
• $850\text{ g}$ into kilograms: g → kg is bigger, so divide. $850 \div 1000 = 0.85\text{ kg}$.
• $7\text{ cm}$ into millimetres: cm → mm is smaller, so multiply. $7 \times 10 = 70\text{ mm}$.

A quick sanity check: if you converted to a smaller unit and your number got smaller, something has gone wrong.

Time is the one measurement that does not use powers of 10, so you have to learn its facts separately:

• $60\text{ seconds} = 1\text{ minute}$
• $60\text{ minutes} = 1\text{ hour}$
• $24\text{ hours} = 1\text{ day}$
• $7\text{ days} = 1\text{ week}$, and about $52\text{ weeks} = 1\text{ year}$

To convert, multiply or divide by the right number — just not by 10. For example, $2.5\text{ hours} = 2.5 \times 60 = 150\text{ minutes}$.

There are two ways to write the time of day:

The 12-hour clock uses am (midnight to noon) and pm (noon to midnight). The 24-hour clock counts the hours straight through from 0000 to 2359. To change a pm time to 24-hour, add 12 to the hours: 3 pm becomes 1500.

When working out how long something lasts, count on in steps — from 1430 to 1615 is 1 hour 45 minutes.

Rulers, weighing scales, measuring jugs and thermometers all have a scale — a row of marks with numbers. Reading one accurately is a careful skill.

Step 1 — Find the value of one small step. Look at two numbered marks, count the small steps between them, and divide.

In the scale above, the marks 0 and 20 have 4 small steps between them, so one step is $20 \div 4 = 5$.

Step 2 — Read the pointer. Count steps from the nearest numbered mark. The pointer sits 2 steps past 60, so it reads $60 + 2 \times 5 = 70$.

Step 3 — Add the unit — grams, millilitres, degrees, whatever the instrument measures.

A good habit is to estimate first: if a jug looks about half full and holds a litre, you expect a reading near 500 ml. A reading of 50 ml would be a warning sign.

Getting confident with units makes lots of later work much smoother:

• Perimeter, area and volume all rely on choosing units and converting them — and on keeping units consistent in a calculation.
• Statistics and data often need conversions so values can be compared fairly.
• Speed, distance and time problems lean directly on units of length and time.
• In everyday life — cooking, travel, shopping, science experiments and sport timing all need confident unit work.

If a measurement question feels tricky later on, it is often a unit conversion underneath that needs a quick refresh. Come back to this guide whenever you need it.