Length Measurements
Primary 4 · Mathematics
Length Measurements
Lesson 2 of 5
25 min
Let's begin
Uwase wants to buy a new pencil case. The shopkeeper says it is "20 cm long". Uwase holds up her hands to guess how long 20 cm is. Can you picture it? In this lesson you will learn the standard units and how big each one is.
The main unit of length is the metre (m). The other units are made from it:
| Unit | Symbol | Size | Use it for |
|---|---|---|---|
| kilometre | km | 1 000 m | distances between towns |
| hectometre | hm | 100 m | (not used much) |
| decametre | dam | 10 m | (not used much) |
| metre | m | 1 m | rooms, cloth, a person's height |
| decimetre | dm | one tenth of a metre | (not used much) |
| centimetre | cm | one hundredth of a metre | books, pencils, your hand |
| millimetre | mm | one thousandth of a metre | very small things, like an ant |
Before you measure, make a sensible guess, called an estimate. Some helpful benchmarks:
So a classroom door is about 2 m tall, not 2 cm and not 20 m.
Put the 0 mark at one end of the object, not the end of the ruler. Then read the number at the other end. The big numbered marks are centimetres, and the small marks between them are millimetres. There are 10 mm in 1 cm.
The pencil reaches 4 small marks past 7, so it is 7 cm 4 mm long. That is the same as 74 mm.
For longer objects, like a desk, use a metre rule: it is 100 cm long and marked in centimetres. We write the length like this: 1 m 20 cm, or 120 cm.
Tip
Start at zero! Always line up the object with the 0 mark on the ruler. Many rulers have a small gap before the 0.
Work each one in your exercise book first. Then tap Show answer to check.
Which unit would you use to measure the distance from Kigali to Huye?
Answer: Kilometres (km).
Which unit would you use to measure the length of your exercise book?
Answer: Centimetres (cm).
Is a classroom door about 2 cm, 2 m or 20 m tall?
Answer: About 2 m.
A crayon starts at 0 and ends 3 small marks after the 9 cm mark. How long is it?
Answer: 9 cm 3 mm, which is 93 mm.
How many millimetres are in 1 cm?
Answer: 10
What you learned today
Sources and credits
The stories, Rwandan worked examples, diagrams and practice questions were written by Ganzaa.
Quick check
Which unit would you use for the distance from Musanze to Kigali?
