Length Measurements

Lessons

Primary 4 · Mathematics

Length Measurements

5 lessons

Lesson 3 of 5

Converting units of length

30 min

Let's begin

Habimana's uncle is a moto driver. His trip from Remera to the city centre is about 6 km. Habimana wants to know how many metres that is. To answer, he needs to change kilometres into metres: this is called converting.

The staircase of units

Each unit is 10 times bigger than the unit just below it.

From a bigger unit to a smaller unit, the number gets bigger: multiply.
From a smaller unit to a bigger unit, the number gets smaller: divide.

The facts to know by heart

Bigger unit	Smaller unit
1 km	1 000 m
1 m	100 cm
1 m	1 000 mm
1 cm	10 mm

Worked examples

6 km to m: 6 × 1 000 = 6 000 m. Habimana's uncle rides 6 000 m.
4 m to cm: 4 × 100 = 400 cm.
2 m 35 cm to cm: 2 m = 200 cm, and 200 + 35 = 235 cm.
560 cm to m and cm: 560 ÷ 100 = 5 remainder 60, so 560 cm = 5 m 60 cm.
8 000 m to km: 8 000 ÷ 1 000 = 8 km.
45 mm to cm and mm: 45 ÷ 10 = 4 remainder 5, so 45 mm = 4 cm 5 mm.
With a decimal: 1.5 km = 1 km + half a km = 1 000 m + 500 m = 1 500 m.

Tip

Big to small, the number grows tall. When you change to a smaller unit, you need more of them, so multiply. When you change to a bigger unit, you need fewer, so divide.

✦

Try it yourself

Work each one in your exercise book first. Then tap Show answer to check.

1

6 km = ___ m

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Answer: 6 000 m

2

7 m = ___ cm

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Answer: 700 cm

3

3 m 25 cm = ___ cm

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Answer: 325 cm

4

480 cm = ___ m ___ cm

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Answer: 4 m 80 cm

5

90 mm = ___ cm

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Answer: 9 cm

Quick check

What you learned today

1 km = 1 000 m, 1 m = 100 cm, 1 m = 1 000 mm and 1 cm = 10 mm.
Bigger unit to smaller unit: multiply. Smaller unit to bigger unit: divide.
Mixed lengths like 2 m 35 cm can be written in one unit: 235 cm.

Sources and credits

Grade 4, Unit 5: Multiplicative Comparison and Measurement, Section B (lessons 7–10: measurement conversion), IM K–5 Math, © 2021 Illustrative Mathematics, licensed under CC BY 4.0. Adapted by Ganzaa.

The stories, Rwandan worked examples, diagrams and practice questions were written by Ganzaa.

1The staircase of units

2The facts to know by heart

Worked examples

Try it yourself

The staircase of units

The facts to know by heart