Choosing metric units for length
21 October 2026
Choosing metric units for length
Understanding metric units is essential for 11+ Maths. Children need to know when to use millimetres, centimetres, metres or kilometres for different measurements.
Why choosing the right unit matters
Using the correct unit makes answers clear and practical. For example, you would not measure the length of a pencil in kilometres or the distance to a nearby town in millimetres.
Key insight: Match the unit to the size
Millimetres (mm) are best for small objects like the thickness of a coin. Centimetres (cm) suit everyday objects, such as a book or a pencil. Metres (m) are right for room lengths or playgrounds. Kilometres (km) are used for longer distances, like a walk to school.
Worked example
A child measures a string and finds it is 4500 millimetres long. Which metric unit is most suitable to record its length?
First, think about the size. 4500 millimetres is quite long for millimetres or centimetres.
Next, convert 4500 millimetres to metres. 1 metre = 1000 millimetres.
4500 ÷ 1000 = 4.5 metres.
The length is 4.5 metres, so metres are the most suitable unit for this measurement.
The correct answer is metres.
Practical tip for parents
Ask your child to estimate first. If the object is smaller than a hand, try millimetres or centimetres. If it is as long as a room or more, think in metres or kilometres.
