Measure & compare length (metric units)
Theory
Choose a metric unit that fits the size of the object, so the number is neither tiny nor huge. Compare lengths only when they are in the same unit.
The metric units of length, from smallest to largest, are millimetres (mm), centimetres (cm), metres (m) and kilometres (km).
Match the unit to the object: mm for a coin, cm for a pencil, m for a pool, km for the distance between towns.
Two lengths can be compared straight away only if they share a unit; otherwise convert one first.
These are the length units and how they connect.
| Unit | Best for | Fact |
|---|---|---|
| mm | coin, nail | \(10\) mm \(=1\) cm |
| cm | pencil, book | \(100\) cm \(=1\) m |
| m | pool, room | \(1000\) m \(=1\) km |
| km | road, trip | — |
How to choose a unit and compare
- Look at the size of the object.
- Pick the unit that gives a tidy number.
- Convert to a common unit before comparing lengths.
A pencil is a few of them long, giving a tidy number like \(15\).
Centimetres are the best choice.
Metres would give a number like \(47\,000\), so use a larger unit.
Kilometres keep it as \(47\) km.
All are in centimetres, so compare the numbers.
| \(108\) | \(>\) | \(90>87>78\) |
\(108\) cm is the longest.
Same unit, so order the numbers.
\(9<19<91\), so the order is \(9\) cm, \(19\) cm, \(91\) cm.
Common pitfalls
Frequently asked questions
How do you choose a unit of length?
Match the unit to the size of the object so the number is tidy: mm for a coin, cm for a pencil, m for a pool and km for long distances.
How do you compare two lengths?
Put them in the same unit, then compare the numbers. If they are already in the same unit, just compare the numbers directly.
How many millimetres are in a centimetre?
There are \(10\) millimetres in \(1\) centimetre, \(100\) centimetres in \(1\) metre and \(1000\) metres in \(1\) kilometre.
Is a bigger number always a longer length?
Only when the units are the same. \(5\) m is longer than \(90\) cm, so the unit must be checked before comparing.