Measuring and Money
Primary 3 · Mathematics
Measuring and Money
Lesson 1 of 3
14 min
Let's begin
Uwase's teacher holds up a long wooden stick. This stick is exactly one metre, she says, and she leans it against the wall.
Suddenly the class is full of questions. How many sticks long is our classroom? How heavy is that sack of beans by the door? How much water fits in the jerrycan?
Today you will learn how to answer all three. Grab a ruler if you have one — and let's measure.
Look at a ruler. The small marks are centimetres. We write them as cm.
A centimetre is about as wide as your little finger. A pencil is about 15 cm long.
For long things — a classroom, a football pitch, how tall you are — centimetres would take forever. So we use metres, written m. That teacher's stick is 1 m.
The rule: 1 m = 100 cm.
Worked example
Keza's desk is 1 m and 30 cm long. How many centimetres is that?
The 1 m is 100 cm. Now add the extra 30 cm.
100 + 30 = 130 cm.
Worked example
(going backwards). A rope is 250 cm long. How many metres and centimetres is that?
Take out full metres. 100 cm is 1 m. 200 cm is 2 m.
That leaves 50 cm over.
So the rope is 2 m 50 cm.
Worked example
(comparing). Mugisha's rope is 3 m. Gatete's rope is 250 cm. Whose rope is longer?
You cannot compare metres with centimetres. First make the units the same.
3 m = 300 cm.
Now compare: 300 cm is more than 250 cm.
Mugisha's rope is longer.
For really long journeys, like Kigali to Musanze, even metres are too small. Then we use kilometres (km), and 1 km = 1000 m.
Mass means how heavy something is.
Light things are measured in grams (g). One bean, one spoon of salt — those are just a few grams.
Heavier things are measured in kilograms (kg). A packet of sugar, a bunch of bananas, a baby.
The rule: 1 kg = 1000 g.
Worked example
A packet of sugar has a mass of 1 kg 500 g. How many grams is that?
The 1 kg is 1000 g. Now add the extra 500 g.
1000 + 500 = 1500 g.
Worked example
(comparing). Mutesi has 800 g of salt. Habimana has 1 kg of salt. Who has more?
Make the units the same: 1 kg = 1000 g.
1000 g is more than 800 g, so Habimana has more.
Worked example
(adding). At the market Umutoni buys 2 kg of rice and 3 kg of beans. What is the total mass?
2 kg + 3 kg = 5 kg. The units are already the same, so you simply add.
Capacity means how much a container can hold.
Small amounts are measured in millilitres (ml). A spoonful of medicine is about 5 ml. A small bottle of soda holds 500 ml.
Bigger amounts are measured in litres (L). A jerrycan, a bucket, a saucepan of water.
The rule: 1 litre = 1000 ml.
Worked example
Ishimwe has two bottles. Each one holds 500 ml. Does he have a whole litre?
500 + 500 = 1000 ml.
And 1000 ml is exactly 1 litre. Yes, he has one full litre.
Worked example
A jerrycan holds 20 litres. Mugisha fills it using a bucket that holds 5 litres. How many buckets does he need?
Count up in fives: 5, 10, 15, 20.
He counted 4 times, so he needs 4 buckets.
Worked example
How many millilitres are in 3 litres?
Each litre is 1000 ml. So count: 1000, 2000, 3000.
3 litres = 3000 ml.
Tip
Kilo means a thousand. Milli means a tiny piece
A kilogram is 1000 grams. A millilitre is one tiny piece of a litre — you need 1000 of them to fill it up.
Watch out for the odd one out: a metre is only 100 centimetres, not 1000!
And the golden rule for comparing: make the units match first. Never put centimetres next to metres and guess.
Work each one out on paper first, then tap Show answer to check.
What you learned today
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