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Every time you check your height, weigh ingredients or time a journey, you are using units. This guide walks you through metric units of length, mass and capacity, converting between them, units of time, and reading scales — one clear step at a time.
Mapped to the Cambridge Lower Secondary Mathematics curriculum framework.
Millimetres, centimetres, metres and kilometres — each step is a power of 10.
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: , and .
Mass uses grams, kilograms and tonnes; capacity uses millilitres and litres.
Mass tells you how heavy something is. The metric units are the gram (g), kilogram (kg) and tonne (t):
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):
Decide if you are going to a bigger or smaller unit, then multiply or divide by the power of 10.
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 .
Step 2 — Decide the direction.
Time does not work in tens — learn the special conversion facts.
Time is the one measurement that does not use powers of 10, so you have to learn its facts separately:
Find the value of one small step, then read where the pointer sits.
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 .
Units underpin perimeter, area, volume and handling real data.
Getting confident with units makes lots of later work much smoother:
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.
Sources: Cambridge Lower Secondary Mathematics curriculum framework (Stage 7). Last reviewed 2026-05-19.
Step-by-step solutions for units of measurement, written exactly the way a tutor would explain them at the board.
Question
Convert into millimetres.
Step-by-step solution
Step 1
Recall the conversion fact: .
Step 2
Millimetres are smaller than centimetres, so the number gets bigger — multiply.
Step 3
Multiply 9 by 10.
Answer
.
Question
A bag of flour has a mass of . Write this in kilograms.
Step-by-step solution
Step 1
Recall the conversion fact: .
Question
A film lasts . How many minutes is that in total?
Step-by-step solution
Step 1
Recall that .
Question
On a jug, the marks 0 and 200 ml have 4 equal steps between them. The water level is 3 steps above the 200 ml mark. How much water is in the jug?
Step-by-step solution
Step 1
Find the value of one small step: the gap from 0 to 200 ml is split into 4 steps.
Question
A roll of ribbon is long. Pieces of are cut from it. How many whole pieces can be cut, and how much ribbon is left over?
Step-by-step solution
Step 1
The units do not match. Convert the roll to centimetres so everything is the same unit.
The important words for units of measurement and what they mean — learn these so you can explain your thinking clearly.
A system of measurement based on powers of 10, used for length, mass and capacity.
The measurement of how long, tall or far something is, measured in units such as mm, cm, m and km.
The measurement of how much matter is in an object, measured in units such as grams, kilograms and tonnes.
The amount of liquid a container can hold, measured in units such as millilitres and litres.
Example
A large bottle of water has a capacity of about 2 litres.
To change a measurement from one unit into another, for example from metres into centimetres.
A small metric unit of length. There are 10 millimetres in 1 centimetre.
A large metric unit of length used for long distances. There are 1000 metres in 1 kilometre.
A metric unit of capacity. There are 1000 millilitres in 1 litre.
A row of marks with numbers on a measuring instrument, used to read off a measurement.
Example
A ruler, a thermometer and a measuring jug all have a scale.
A way of writing the time that counts the hours straight through from 0000 to 2359, with no am or pm.
Example
3 pm is written as 1500 on the 24-hour clock.
A sensible, rough value for a measurement, used to check whether an answer is reasonable.
A large metric unit of mass. There are 1000 kilograms in 1 tonne.
The slip-ups students most often make with units of measurement — and simple ways to avoid them.
Why it happens
The conversion fact is known, but the direction is not checked first.
How to avoid it
Ask: 'Is the new unit smaller or bigger?' Smaller unit means a bigger number, so multiply; bigger unit means divide.
Why it happens
The mm-to-cm step is 10, so the same number gets reused for cm-to-m.
How to avoid it
Remember each link separately: but .
Why it happens
Time written as 1:30 looks like the decimal 1.3.
How to avoid it
There are 60 minutes in an hour, not 100. 30 minutes is half an hour, so 1 hour 30 minutes is 1.5 hours.
Why it happens
Marks look evenly spaced, so they are read as single units.
How to avoid it
Count the steps between two numbered marks and divide to find the value of one step.
Why it happens
The numbers are there, so the units are added without checking.
How to avoid it
Convert everything to the same unit before adding, subtracting or dividing.
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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.
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, , and .
Step 3 — Carry out the calculation.
A quick sanity check: if you converted to a smaller unit and your number got smaller, something has gone wrong.
To convert, multiply or divide by the right number — just not by 10. For example, .
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.
Step 2 — Read the pointer. Count steps from the nearest numbered mark. The pointer sits 2 steps past 60, so it reads .
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.
Step 2
Kilograms are bigger than grams, so the number gets smaller — divide.
Step 3
Divide 1500 by 1000.
Answer
.
Step 2
Convert the 2 hours into minutes by multiplying.
Step 3
Add the extra 15 minutes.
Answer
The film lasts .
Step 2
Each step is 50 ml. The level is 3 steps above 200 ml.
Step 3
Add this to the 200 ml mark.
Answer
There are of water in the jug.
Step 2
Divide the total length by the length of one piece.
Step 3
Only whole pieces count, so 8 pieces can be cut. Work out the ribbon those pieces use.
Step 4
Subtract to find the leftover ribbon.
Answer
8 whole pieces can be cut, with of ribbon left over.