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How to convert units: metres, centimetres, kilometres, grams, kilograms and litres
A simple rule for converting metres, centimetres, kilometres, grams, kilograms, litres and millilitres, with a conversion table, worked examples and the common mistakes.
Converting units is one of the most useful skills in primary maths, and it comes back in science and everyday life. There is one rule that does most of the work.
The one rule
- Going to a smaller unit (metres to centimetres): there are more of them, so multiply.
- Going to a bigger unit (grams to kilograms): there are fewer of them, so divide.
Ask yourself first: should my number get bigger or smaller? Then you know whether to multiply or divide.
The facts to remember
| Measure | Facts |
|---|---|
| Length | 1 cm = 10 mm · 1 m = 100 cm · 1 km = 1,000 m |
| Mass | 1 kg = 1,000 g |
| Capacity | 1 L = 1,000 mL |
| Time | 1 hour = 60 minutes · 1 minute = 60 seconds |
Worked examples
- 3 m to cm. Smaller unit, so multiply: 3 × 100 = 300 cm.
- 2 km to m. 2 × 1,000 = 2,000 m.
- 4,000 g to kg. Bigger unit, so divide: 4,000 ÷ 1,000 = 4 kg.
- 300 cm to m. 300 ÷ 100 = 3 m.
- 2.5 km to m. 2.5 × 1,000 = 2,500 m.
- 0.5 L to mL. 0.5 × 1,000 = 500 mL.
- 3.2 m to cm. 3.2 × 100 = 320 cm.
Mistakes to avoid
- Multiplying when you should divide. Check the direction: converting 4,000 g to kg should give a smaller number.
- Using the wrong fact. There are 100 cm in a metre but 1,000 m in a kilometre. Say the fact aloud before using it.
- Moving the decimal point the wrong way. Multiplying by 100 moves the point two places to the right.
- Mixing units in one sum. Change everything to one unit first, then add or compare.
A quick way with place value
Multiplying by 10, 100 or 1,000 moves each digit one, two or three places to the left, and dividing moves it to the right. If you understand place value, conversions become a matter of counting places.
Practise it
- Unit conversions practice online with instant feedback.
- Unit conversion worksheets with an answer key.
- Hours and minutes practice for time units.
Common questions
How many centimetres are in a kilometre?
There are 100 cm in a metre and 1,000 m in a kilometre, so 1 km = 100,000 cm.
How many millilitres are in 2.5 litres?
2.5 × 1,000 = 2,500 mL.
Why do we multiply when we go to a smaller unit?
A smaller unit fits into the measure more times. A metre holds 100 centimetres, so 3 metres hold 300 of them.
Which grade learns this?
Schools usually teach it between Grade 3 and Grade 5 (Class 3 to 5), but the order differs by school and country.
This guide is general information. For exams, follow your school, teacher and official exam board, and use your own judgement about what suits your child.