Units of length
We measure length with millimetres (mm), centimetres (cm), metres (m) and kilometres (km). We pick the unit that fits the object: the thickness of a coin uses mm, the length of a pencil uses cm, the height of a door uses m, and the distance between two towns uses km.
To swap between units we only need to multiply or divide. Learn these three facts by heart and every conversion becomes easy.
Key idea
1 cm = 10 mm 1 m = 100 cm 1 km = 1000 m.
To go from a big unit to a small unit, multiply. From a small unit to a big unit, divide.
Worked example
Worked example
Change 2.5 m into centimetres.
Metres to centimetres is big → small, so multiply by 100.
2.5 × 100 = 250 cm.
Check: 250 ÷ 100 = 2.5 m. Correct.
When you add or subtract lengths, keep the same units under each other, just like adding money.
Remember
- Bigger unit → smaller unit: multiply.
- Smaller unit → bigger unit: divide.
- Always write the correct unit in your answer.