Units of length
A grain of rice is a few millimetres long. Your finger is a few centimetres long. Your bed is about 2 metres long. The road to the next town is a few kilometres long. Each unit is built for a certain size.
What is this?
The four standard units of length we use are:
- Millimetre (mm) , the smallest. About the thickness of a fingernail.
- Centimetre (cm) , width of a button.
- Metre (m) , height of a small child.
- Kilometre (km) , distance you walk in about 12 minutes.
They are connected by the number 10 and 1000:
- 1 cm = 10 mm
- 1 m = 100 cm = 1,000 mm
- 1 km = 1,000 m
So if a pencil is 12 cm long, that is the same as 120 mm. And a 2 m ribbon is 200 cm or 2,000 mm.
To go from a bigger unit to a smaller unit, we multiply. From a smaller unit to a bigger unit, we divide.
- 5 m = 5 × 100 = 500 cm.
- 4,500 mm = 4,500 ÷ 10 = 450 cm.
Pick the right unit for the right job. We don't measure a road in millimetres or a needle in kilometres. The right unit gives a small, easy number.
Worked examples
Example 1. Convert 3 m to cm.
- 1 m = 100 cm.
- 3 m = 3 × 100 = 300 cm.
Example 2. Convert 2,500 m to km.
- 1 km = 1,000 m.
- 2,500 ÷ 1,000 = 2.5 km, or 2 km 500 m.
Example 3. Convert 75 mm to cm.
- 1 cm = 10 mm.
- 75 ÷ 10 = 7.5 cm, or 7 cm 5 mm.
Try it
- Convert 6 m to cm.
- Convert 4 km to m.
- Convert 250 cm to m.
- Convert 35 mm to cm.
- Pick the best unit: thickness of a coin, height of a tree, distance between two cities, length of an ant.
Hands-on
Ruler walk. Find 5 objects in your room. Measure them with a ruler in cm. Write each in mm too. (Multiply by 10.)
One-kilometre walk. Ask a grown-up to help you find a 1-km road stretch (your school path, a market path). Walk it at a normal pace and time yourself. Does it match the "12 minutes" rule of thumb?