Key stage 2 maths, ages 9 to 11, about 15 minutes
Measures and Converting Units
Change between kilometres, metres, centimetres and millimetres, and between metric and imperial units.
The metric ladder
Pick a starting measurement, then drag or tap the ladder to choose the unit to convert to.
2.5 km = 2,500 m
Metres are smaller than kilometres, so the number gets bigger. Multiply by 1,000: every digit moves 3 places to the left.
Zoom the ruler
Zoom from 10 kilometres down to 1 centimetre. The marks change unit as you go.
Each unit keeps its colour everywhere in this lesson: kilo green, metres, grams and litres blue, centi amber, milli pink.
Ten questions, each in steps. A question counts only if every step is right first time. New numbers every time.
Why this mission needs it: Drawings, fuel gauges and maps use millimetres, metres, kilometres, kilograms and litres: converting cleanly between units prepares you for physics formulas.
The idea
- Kilometre, 1 km = 1,000 m
- For long distances, such as from one town to the next.
- Metre, 1 m = 100 cm
- About the length of a big step.
- Centimetre, 1 cm = 10 mm
- About the width of your little finger.
- Millimetre, the gap between the smallest marks on a ruler
- About the thickness of a bank card.
Same length, different number
A door 2 m tall is also 200 cm tall. The door has not changed; only the unit has. A centimetre is much smaller than a metre, so it takes more of them to measure the same door.
Key point: Changing to a smaller unit makes the number bigger. Changing to a bigger unit makes the number smaller.
The metric ladder
Each step down the ladder, from km to m, from m to cm and from cm to mm, goes to a smaller unit, so you multiply: by 1,000, by 100 and by 10. Each step back up goes to a bigger unit, so you divide by the same number.
Key point: Before you calculate, ask: is the new unit smaller or bigger? Smaller means multiply; bigger means divide.
Mass and capacity work the same way
Mass is how heavy something is, and capacity is how much a container holds. Kilo always means 1,000, centi means one hundredth and milli means one thousandth. So 1 kilogram is 1,000 grams, just as 1 kilometre is 1,000 metres, and 1 litre is 1,000 millilitres, just as 1 metre is 1,000 millimetres.
Key point: Learn what kilo, centi and milli mean, and you know the factors for length, mass and capacity.
Imperial units
Before metric units, Britain used imperial units: inches and miles for length, pounds for mass and pints for capacity. Some are still used every day, such as miles on road signs and pints of milk.
Key point: For working in your head, use rough equivalents: 1 inch is about 2.5 cm, 1 kg is about 2.2 pounds, 1 pint is a little more than half a litre, and 5 miles is about 8 km.
Check that the answer makes sense
A mile is longer than a kilometre, so a distance in kilometres is always a bigger number than the same distance in miles. After any conversion, check that the number has moved the right way.
Key point: If the new unit is smaller, the number should have gone up. If it has gone down, you have divided when you should have multiplied.
Where you’ll see this: UK road signs give distances in miles and speed limits in miles per hour. In France, and most other countries, they are in kilometres, so drivers who take a car across the Channel have to convert.
Worked example
The International Space Station flies about 400 km above the Earth. How high is that in metres, and roughly how high in miles?
- 1Metres are smaller than kilometres, so the number gets bigger: multiply by 1,000. , so that is 400,000 m.
- 2For miles, use 5 miles is about 8 km. Find how many lots of 8 km there are in 400 km: .
- 3Each lot of 8 km is about 5 miles: .
- 4Check: a mile is longer than a kilometre, so the number of miles should be smaller than 400. It is.
Answer
Practice
Practise with the step-by-step quiz in the Try it panel. Each question is answered one step at a time, with fresh numbers every time, and the questions you miss come back for review.
Notes
Measures and converting units
In the last lesson you multiplied and divided by 10, 100 and 1,000. That is all the maths you need here. A measurement is a number and a unit together. The same length can be written with different units: 1 metre is also 100 centimetres. Converting changes the number and the unit, never the length itself.
Units of length
| Unit | Symbol | How big |
|---|---|---|
| kilometre | km | 1 km = 1,000 m |
| metre | m | 1 m = 100 cm |
| centimetre | cm | 1 cm = 10 mm |
| millimetre | mm | the gap between the smallest marks on a ruler |
Moving up and down the ladder
Going to a smaller unit, multiply. Going to a bigger unit, divide.
- 6 km to metres: metres are smaller, so multiply by 1,000. , so 6 km = 6,000 m.
- 250 cm to metres: metres are bigger, so divide by 100. , so 250 cm = 2.5 m.
- 4.2 cm to millimetres: millimetres are smaller, so multiply by 10. , so 4.2 cm = 42 mm.
Multiplying by 10 moves every digit one place to the left; dividing by 10 moves every digit one place to the right. For 100 the digits move two places, and for 1,000 they move three.
Jumping more than one step
From metres to millimetres is two steps down: and then , which is altogether. So 3 m = 3,000 mm. A common mistake is to use 100 here, because 100 cm make a metre. Count the steps on the ladder before you choose the number.
Mass and capacity
Mass is how heavy something is. Capacity is how much a container holds.
| Measure | Units | How big |
|---|---|---|
| Mass | kilogram (kg) and gram (g) | 1 kg = 1,000 g |
| Capacity | litre (l) and millilitre (ml) | 1 l = 1,000 ml |
They work like length. 3.4 kg = 3,400 g, and 750 ml = 0.75 l.
Up to three decimal places
Dividing by 1,000 can give three decimal places. A path 1,234 m long is 1.234 km. A 5 m rope is 0.005 km: the 5 moves three places to the right, so put in zeros to hold the places.
Imperial units
Imperial units are older British units. Some are still in everyday use in the UK.
| Imperial unit | Roughly in metric |
|---|---|
| 1 inch | 2.5 cm |
| 1 mile | 1.6 km |
| 1 pound (lb) | 0.45 kg, so 1 kg is about 2.2 pounds |
| 1 pint | 0.57 litres, a little more than half a litre |
These are rounded, which is fine for estimating. Some imperial units are now defined exactly by metric ones: 1 inch is exactly 2.54 cm, 1 mile is exactly 1.609344 km, and 1 pound is exactly 0.45359237 kg.
Miles and kilometres
5 miles is about 8 km. To convert, find how many lots of 5 or 8 there are.
- 20 miles to kilometres: lots, and , so about 32 km.
- 40 km to miles: lots, and , so about 25 miles.
Does the answer make sense?
After every conversion, check the direction. A kilometre is shorter than a mile, so the kilometre number should always be the bigger one. A kilogram is heavier than a pound, so the pound number should be the bigger one.
Next on the journey
That completes your toolkit. Scientists use many more units than these, with prefixes such as mega and nano, and units made from two units, such as kilometres per hour. Mission 2, "Get to orbit", starts with converting units like these.
Sources
The topics in these notes follow the Department for Education document below.
- National curriculum in England: mathematics programmes of study (Department for Education)
Where this fits in the curriculum
Key stage 2 (Years 5 and 6) in the curriculum for England.
The Department for Education says pupils should be taught to:
Year 5
“convert between different units of metric measure [for example, kilometre and metre; centimetre and metre; centimetre and millimetre; gram and kilogram; litre and millilitre]”
Year 5
“understand and use approximate equivalences between metric units and common imperial units such as inches, pounds and pints”
Year 6
“convert between miles and kilometres”
Year 6
“use, read, write and convert between standard units, converting measurements of length, mass, volume and time from a smaller unit of measure to a larger unit, and vice versa, using decimal notation to up to 3 decimal places”
Quoted from National curriculum in England: mathematics programmes of study (Department for Education, gov.uk), checked on 12 October 2026. This is the content every exam board has to cover. It is not one board’s specification, so use your own for exactly what is examined.
Tick it off once the practice questions make sense.
Next mission: Get to orbit
Free to use, not free to run: if this lesson helped you, your child or your class, chip in towards hosting and new lessons, or help review a mission.
Chip in on Ko-fiThis lesson was written with AI assistance. Check formulas and figures against your textbook or exam board’s specification before relying on them in assessed work.