Maths
How do you measure length?
Quick answer
Length is how long, tall or far something is. It is measured with a ruler or tape in units such as millimetres, centimetres, metres and kilometres (or inches, feet and miles).
In one sentence
Length tells you how long, wide, tall or far something is, and you measure it in units like centimetres and metres.
Explained
Long ago people measured with their bodies: hand spans, feet and paces. The trouble is that everyone's feet are different. So we use standard units that mean the same everywhere.
giraffe
snail
ruler
Metric units are used in most of the world and in science:
| Unit | Short | Size |
|---|---|---|
| millimetre | mm | about the thickness of a bank card |
| centimetre | cm | 10 mm, about the width of a fingernail |
| metre | m | 100 cm, about one big stride for an adult |
| kilometre | km | 1000 m, about a 12-minute walk |
The metric system is built on tens, so converting is a matter of multiplying or dividing by 10, 100 or 1000. To change metres to centimetres, multiply by 100: 1.6 m = 160 cm. To change cm to m, divide by 100: 45 cm = 0.45 m. This is place value at work.
Imperial units are still common in the US and in everyday UK life (road signs in miles, height in feet):
- 12 inches = 1 foot
- 3 feet = 1 yard
- 1,760 yards = 1 mile
- 1 inch = 2.54 cm exactly, and 1 mile is about 1.6 km
Using a ruler. Put the zero mark (not the end of the ruler, which is often a little before zero) at one end of the object. Keep the ruler straight and read where the other end lands. If you can't start at zero, subtract: from the 3 cm mark to the 11 cm mark is 8 cm.
Choose the right tool and unit. A ruler suits a pencil, a tape measure suits a room, and a map or a car's milometer suits the distance between towns.
Measuring all the way round a shape gives its perimeter.
Examples
- 35 mm = 3.5 cm
- 2 m 40 cm = 240 cm
- 3,200 m = 3.2 km
- A 30 cm ruler is about 1 foot (12 in).
- A 5 km run is about 3.1 miles.
Common mistakes
- Starting at the end of the ruler instead of at 0. Fix: line up the 0 mark.
- Mixing units when adding: 1 m + 20 cm is 120 cm, not 21.
- Forgetting the unit. "It's 12" could mean 12 cm or 12 m.
Quick quiz
Length and measuring
Check what you know with 5 quick questions.
Question 1 of 5
How many centimetres are in 1 metre?
Practise it
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Browse maths worksheets and look for measuring and length pages.
For grown-ups
Related topics
Perimeter
Perimeter is the total distance all the way around the outside of a flat shape. For a rectangle 5 cm long and 3 cm wide, it is 5 + 3 + 5 + 3 = 16 cm.
Decimals
Decimals are numbers with a decimal point. Digits after the point show tenths, hundredths and so on, so 2.5 means two and five tenths.
Mass and weight
Mass is how much stuff is in an object, measured in grams and kilograms. Weight is the pull of gravity on that mass. In everyday life people say weight for both.
Capacity
Capacity is how much a container can hold, usually measured in millilitres (ml) and litres (l). A large bottle of water often holds 2 litres, which is 2000 ml.
Estimation
Estimating means making a sensible guess at an answer, usually by rounding the numbers first. It tells you roughly what to expect and helps you spot mistakes.
2D shapes
2D shapes are flat shapes with length and width but no thickness, like circles, triangles, squares and hexagons. You can draw them on paper.