Capacity Measurements
Primary 4 · Mathematics
Capacity Measurements
Lesson 1 of 4
15 min
Let's begin
It is Saturday morning in Nyamata. Uwase and Mugisha must fetch water for the family. Uwase carries a fat yellow jerrycan. Mugisha carries a tall, thin bucket.
"My bucket is taller, so it holds more," says Mugisha. Uwase is not so sure.
How can they find out who carries more water? Today you will learn to measure how much a container can hold.
The capacity of a container is the amount of liquid it holds when it is full. A cup, a bottle, a jerrycan and a water tank all have a capacity.
A tall container does not always hold more. A short, wide basin can hold more than a tall, thin bottle. To be sure, we must measure.
Uwase takes a cup. She counts how many cups of water fill each container.
80 − 48 = 32, so the jerrycan holds 32 cups more than the bucket. Uwase was right!
Careful! Cups are not all the same size. If Mugisha used a bigger cup, he would count fewer cups for the same bucket. Two families could get two different answers. That is why we need a unit that is the same everywhere.
The litre is the standard unit of capacity. We write it with a capital L. A litre is the same size in Kigali, in Rusizi and all over the world.
An estimate is a clever guess. Before you measure, think of three numbers: one that is too low, one that is too high and one that is about right.
Worked example
About how many litres does a bucket hold?
Too low: 2 L. Too high: 50 L. About right: 10 L.
Now check it. Pour in water from a 1 L bottle, one litre at a time. Draw a mark at the new water level each time. A measuring jug already has these marks.
To read a measuring jug, look straight at the top of the water. Find the mark it reaches. This jug has 3 L of water in it. It can hold 5 L when it is full, so its capacity is 5 L.
Tip
Tall is not always more. Your eyes can trick you. Pour and count, or read the litre marks. Always say the unit: "20 L", not just "20".
Work each one in your exercise book first. Then tap Show answer to check.
A basin holds 36 cups of water. A cooking pot holds 29 of the same cups. Which holds more, and how many cups more?
Answer: The basin holds 7 cups more, because 36 − 29 = 7.
Which is the best estimate for the capacity of a yellow jerrycan: 2 L, 20 L or 200 L?
Answer: 20 L. 2 L is only two small bottles, and 200 L would be far too heavy to carry.
Keza fills an empty basin with a 1 L bottle. She pours in 7 full bottles and then the basin is full. What is the capacity of the basin?
Answer: 7 L. Each bottle adds 1 L.
Mugisha fills a bucket with 12 of his cups. Gatete fills the same bucket with 8 of his cups. Whose cup is bigger?
Answer: Gatete's cup. He needs fewer cups to fill the same bucket, so each of his cups holds more.
A measuring jug is marked 1 L, 2 L, 3 L and 4 L. The water is exactly halfway between 2 L and 3 L. How much water is in the jug?
Answer: 2 12 L, which is two and a half litres.
What you learned today
Sources and credits
The stories, Rwandan examples, drawings and practice questions were written by Ganzaa.
Quick check
A taller container always holds more water.
