Measuring length: from hand spans to meters
๐ In short
Long ago people measured with hand spans, cubits and feet. But a tall man's span is bigger than a child's, so the measurements differed! So the world agreed on one unit, the โmeterโ, along with the centimeter and the kilometer.
๐คฉ Did you know?
- France adopted the meter in 1795, and it then spread to most countries in the world.
- The longest river in the world, the Nile, is about 6650 kilometers long.
- A big giraffe is about 5 meters tall.
- Your little fingernail is about 1 centimeter wide: a ruler you always carry!
๐ Find out more
๐ Body measures
A hand span is the distance from the tip of the thumb to the tip of the little finger with the hand open, and a cubit is from the elbow to the fingertips. Builders and merchants used them for centuries, and we still use them to estimate lengths quickly.
๐ The meter and its family
A meter is about one big step. We split it into 100 centimeters (cm), and a centimeter into 10 millimeters (mm). A thousand meters make a kilometer (km), used to measure distances between towns.
๐ก How do we measure with a ruler?
Put the ruler's zero exactly at the start of the object, then read the number at its end. If we don't start at zero, we subtract: from 3 to 10 the length is 10 โ 3 = 7 cm.
๐ฏ Which unit should we pick?
We measure an insect in millimeters, a pencil in centimeters, a room in meters and the road between two towns in kilometers. Picking the right unit keeps the numbers easy.
๐ See it in a picture
๐งฎ Worked examples
- 1 m = 100 cmOne meter is a hundred centimeters.
- 1 km = 1000 mOne kilometer is a thousand meters.
- 2 m and 35 cm = 235 cm200 + 35.
๐ Key words
- Meter
- The basic unit for measuring length
- Centimeter
- A hundredth of a meter, written cm
- Kilometer
- A thousand meters, written km
๐งช Try it at home
Measure the kitchen table with your hand span, then ask one of your parents to measure it with theirs. Why is the number different? Then measure it in centimeters with a ruler or tape measure.
๐งฉ Test yourself
1 How many centimeters are in a meter?
2 What unit do we use for the distance between two towns?
3 A pencil starts at 2 and ends at 9 on the ruler. How long is it?