Measuring length
Length tells us how long something is. We use units like millimetre (mm), centimetre (cm), metre (m) and kilometre (km).
Key idea
1 cm = 10 mm · 1 m = 100 cm · 1 km = 1000 m. Bigger unit to smaller: multiply.
Changing units
Use small units for small things (a pencil in cm) and big units for long distances (a road in km).
Worked example
How many centimetres are in 3 metres? Since 1 m = 100 cm, we multiply: 3 × 100 = 300 cm.
To change a smaller unit into a bigger one, we divide. For example, 250 cm in metres: 250 ÷ 100 = 2 m 50 cm. We can also add lengths. A rope of 1 m 40 cm joined to one of 60 cm gives 140 cm + 60 cm = 200 cm = 2 m. Always use the same unit before adding.
Remember
- 1 m = 100 cm.
- 1 km = 1000 m.
- Bigger to smaller: multiply; smaller to bigger: divide.