How tall are you? How far is your school from home? How long is your pencil? We need to measure length to answer these questions. Length tells us how long, how wide, or how tall something is.
Units of Length
We use standard units so that everyone gets the same measurement.
- millimetre (mm) — very small; thickness of a coin
- centimetre (cm) — length of your fingernail; a ruler shows cm
- metre (m) — height of a door; length of a classroom
- kilometre (km) — distance between cities
Key Conversions
| Conversion | Fact |
|---|---|
| 1 cm = 10 mm | 10 millimetres make 1 centimetre |
| 1 m = 100 cm | 100 centimetres make 1 metre |
| 1 km = 1,000 m | 1,000 metres make 1 kilometre |
How to Use a Ruler
- Place the 0 mark of the ruler at one end of the object.
- Read the measurement at the other end.
- Always record the unit (e.g., 7 cm, not just 7).
Converting Units
To convert larger to smaller units — multiply.
To convert smaller to larger units — divide.
Convert 3 m to cm.
3 m = 3 x 100 = 300 cm.
Convert 5 km to m.
5 km = 5 x 1,000 = 5,000 m.
Convert 250 cm to m.
250 cm = 250 / 100 = 2 m 50 cm or 2.5 m.
A garden path is 4 m 35 cm long. Express this in cm.
4 m = 400 cm. Total = 400 + 35 = 435 cm.
Add 2 m 45 cm and 1 m 65 cm.
cm: 45 + 65 = 110 cm = 1 m 10 cm. Carry 1 m.
m: 2 + 1 + 1 = 4 m.
Total = 4 m 10 cm.
Estimating Length
- Estimation means making a sensible guess without measuring exactly.
- Width of a finger ≈ 1 cm
- Length of a class blackboard ≈ 4 m
- Distance from Delhi to Mumbai ≈ 1,400 km
Common mistakes
- Forgetting to write the unit after the number.
- Mixing up cm and mm — always check the conversion.
- When adding lengths, first convert to the same unit.
Summary
Length is measured in mm, cm, m, and km. 1 cm = 10 mm, 1 m = 100 cm, 1 km = 1,000 m. Multiply to convert larger to smaller units; divide to convert smaller to larger units.