Length Measurements
Primary 4 · Mathematics
Length Measurements
Lesson 1 of 5
20 min
Let's begin
Mugisha and his little sister Keza measure their classroom by walking across it. Mugisha counts 12 steps. Keza counts 16 steps. The classroom did not change size, so who is right?
Length tells us how long, how tall, how wide or how far something is. The height of a pupil, the width of a door and the distance from home to school are all lengths.
Long ago, and still today when we have no ruler, people measured with their bodies:
These are called non-standard units, because they are not the same for everyone.
A father's big hand measures the bench as 4 spans, but Keza's small hand measures it as 6 spans. Both are right for their own hands! In the same way, Keza's steps are shorter than Mugisha's, so she needs more of them to cross the classroom.
To buy cloth at the market or to build a house, everyone must get the same answer. So the whole world agreed on standard units: the metre, the centimetre, the kilometre and others. A metre is the same length in Rwanda, in Kenya and everywhere else.
| Instrument | Good for measuring |
|---|---|
| Ruler (15 cm or 30 cm) | small things: a pencil, a book, a line in your exercise book |
| Metre rule | a desk, a door, a blackboard |
| Tape measure | a person's height, the classroom, a football pitch |
| Measuring wheel (trundle wheel) | long distances, like the length of a road |
Tip
Your body is a ruler that changes. Hand spans and steps are handy for guessing, but only standard units give everyone the same answer.
Work each one in your exercise book first. Then tap Show answer to check.
Which instrument would you use to measure the length of a football pitch?
Answer: A tape measure (or a measuring wheel).
Which instrument would you use to measure your pencil?
Answer: A ruler.
Ishimwe and Mutesi each measure the same desk in hand spans. Ishimwe gets 5 and Mutesi gets 7. Why are the answers different?
Answer: Their hands are different sizes. Mutesi's hand span is smaller, so she needs more spans.
A table is 5 of Gatete's hand spans long, and his hand span is 20 cm. How long is the table?
Answer: 100 cm, which is 1 m (5 × 20 = 100).
What do we call the distance you cover in one long step?
Answer: A stride (or a pace).
What you learned today
Sources and credits
Written by Ganzaa. The story, diagram and practice questions are original.
Quick check
Which instrument is best for measuring a door?
