Measuring and converting
We measure length in kilometres (km), metres (m) and centimetres (cm); mass in kilograms (kg) and grams (g); and volume of liquid in litres (ℓ) and millilitres (mℓ). Choosing the right unit matters: we measure a pencil in cm but a road in km. To change a bigger unit into a smaller one we multiply; to change a smaller unit into a bigger one we divide.
Key idea
1 m = 100 cm · 1 km = 1000 m · 1 kg = 1000 g · 1 ℓ = 1000 mℓ.
A worked example
Worked example
Change 2500 mℓ into litres. Since 1 ℓ = 1000 mℓ, we divide: 2500 ÷ 1000 = 2.5 ℓ. To check, multiply back: 2.5 × 1000 = 2500 mℓ. Correct!
Remember
- Big unit → small unit: multiply by 1000 (or 100 for m to cm).
- Small unit → big unit: divide.
- Add or subtract only when both amounts use the same unit.
- When you multiply or divide by 1000, move the decimal point three places in the right direction.