Skip to content

Kilograms and litres

Measurement · level 2, lesson 3 of 6

A question to start

Put a 1 litre bottle of water on one side of a balance, and a 1 kilogram bag of flour on the other side. Which side goes down?

1 L 1 kg

Neither! The balance stays (very nearly) level. A litre of water weighs about one kilogram.

That is not luck. When scientists in France invented these units in the 1790s, they made it that way: they chose the kilogram to be the weight of one litre of water. So a bottle of water is a "pocket kilogram" you can find in almost any kitchen. In this lesson you will meet the kilogram and the litre, and learn to read the scales and jugs that measure them.

The idea

Two different questions

When you hold a full jug of juice, you can ask two different questions.

  • How heavy is it? That is its mass (in everyday talk, its weight). We measure it in kilograms.
  • How much does it hold? That is its capacity. We measure it in litres.

These are different! A big box of popcorn holds a lot but is light. A small jar of coins holds a little but is heavy. You saw this in level 1 with the balloon and the stone.

The kilogram

The kilogram is the standard unit for how heavy things are. We write kg. Just like the metre, it is the same everywhere in the world.

Benchmarks to remember:

  • a 1 litre bottle or carton of water or milk: about 1 kg,
  • a bag of sugar or flour: often 1 kg or 2 kg (look on the bag),
  • a pineapple: about 1 kg,
  • a big dictionary: about 1 or 2 kg,
  • many 8-year-olds: about 25 kg.

Small things are much lighter than a kilogram. A paper clip is a tiny part of a kilogram. For those, we use a smaller unit, the gram (g). 1000 grams make 1 kilogram, and a paper clip is about 1 gram. You will use grams a lot in level 3.

Weighing with a balance and kilogram weights

A balance compares two things. To find out how many kilograms something weighs, put it on one side and put kilogram weights on the other side until the balance is level. If it balances 3 kilogram weights, it weighs 3 kg. This is exactly the block method from level 1, but now every "block" is a kilogram, the same block for everyone.

Reading a scale

A scale with a dial has a needle (a pointer) that turns as you put things on it. The dial has numbers and marks like a curved ruler.

0 1 2 3 4 5 kg 3 kg 0 1 2 3 4 5 kg ?

On the first scale, the needle points right at 3. The bag weighs 3 kg.

On the second scale, the needle is between 1 and 2. Look at the marks. Between each number there is one small mark, exactly halfway. The needle is on that small mark. So the bag weighs 1 and a half kilograms.

How do you know the small mark means a half? Count the spaces between 1 and 2. There are 2 equal spaces. So each space is one half of a kilogram. Always count the spaces between two numbers before you read a scale: on some scales there are 2 spaces, on some 4, on some 10.

Many scales today are digital: they just show the number. A bathroom scale might show "25 kg". Easy to read, but you still need to know what the number means.

The litre

The litre is the standard unit for how much a container holds. We write L (a capital L, so it does not look like the number 1).

Benchmarks:

  • a carton of milk or juice: often 1 L,
  • a big bottle of pop: 2 L,
  • in parts of Canada, milk is sold in 4 L bags (three smaller bags inside one big bag),
  • a bucket: about 10 L,
  • a bathtub: well over 100 L.

A small cup holds much less than a litre. Four or five small cups of water usually fill a litre.

Reading a measuring jug

A measuring jug has marks up its side, like a ruler standing up.

1 L 2 L jug A 1 L 2 L jug B

Jug A has water up to the 1 L mark. It holds 1 litre of water.

Jug B has water to the small mark between 1 L and 2 L. There are 2 spaces between 1 and 2, so the small mark is halfway. Jug B holds 1 and a half litres.

To read a jug well:

  1. Put it on a flat table, not tipped.
  2. Bend down so your eyes are level with the top of the water. Looking from above or below makes the water seem higher or lower than it is.
  3. Count the spaces between the numbers before you read.

A litre of water is about a kilogram

Back to the question at the start. Why does a litre of water balance a kilogram? Because the people who invented the units chose it. The first kilogram was defined as the weight of 1 litre of water. (Today the kilogram is defined in a fancier way, which you will meet much later, but the old link still works for everyday use.)

This gives you a great trick: for water, litres and kilograms go together.

  • 1 L of water weighs about 1 kg.
  • 2 L of water weighs about 2 kg.
  • A 10 L bucket of water weighs about 10 kg. That is why a full bucket is so hard to carry!

But this trick is only for water (and things close to water, like milk and juice). A litre of popcorn weighs much less than a kilogram, because it is mostly air. A litre of honey weighs more than a kilogram, because honey is packed tighter than water.

Mass on Mars

If you stood on a bathroom scale on Mars, it would show a much smaller number: a bit more than a third of what it shows on Earth. A child who reads 25 kg at home would read only about 9 or 10 kg on Mars. Did the child lose weight? No! The child is made of exactly the same amount of stuff. Mars just pulls more gently, so the scale's spring is squashed less. A balance with kilogram weights would still say 25 kg on Mars, because Mars pulls more gently on the weights too. Scientists say the child's mass is still 25 kg everywhere.

Worked examples

Example 1. Read the scale. A melon is put on a scale.

0 1 2 3 4 5 kg melon
  1. The needle is between 3 and 4.
  2. There are 2 spaces between 3 and 4, so the small mark is a half.
  3. The needle is on the small mark. The melon weighs 3 and a half kilograms.

Example 2. Litres and kilograms of water. A fish tank holds 20 L of water. About how heavy is the water?

  1. 1 L of water weighs about 1 kg.
  2. So 20 L of water weighs about 20 kg.
  3. That is about as heavy as a 6-year-old child. This is why you should never try to lift a full fish tank.

Example 3. How many cartons? A recipe for punch needs 3 L of juice. Juice comes in 1 L cartons. How many cartons do you need? And if the juice came in 2 L bottles?

  1. Each carton is 1 L. 3 L needs 3 cartons.
  2. Each bottle is 2 L. One bottle is 2 L: not enough. Two bottles are 4 L: enough, with 1 L left over.
  3. So you need 2 bottles (and you will have 1 L to spare).

Example 4. Kilogram or litre? Which unit would you use: the flour for a cake, the water in a pool, the weight of a dog, the milk in a carton?

  1. Flour, how heavy: kilograms (or grams for a small amount).
  2. Water in a pool, how much it holds: litres (a lot of them).
  3. A dog, how heavy: kilograms.
  4. Milk in a carton, how much it holds: litres.

Try it

  1. Which is heavier: 1 kg of feathers or 1 kg of stones?
  2. A scale goes from 0 to 5 kg with one small mark halfway between each number. The needle is on the small mark between 4 and 5. What does the thing weigh?
  3. About how heavy is 3 L of water?
  4. Would you measure the water in a bath in kilograms or litres? The weight of a cat?
  5. A jug has 2 L of water. You pour out half a litre. How much is left?
  6. Pop comes in 2 L bottles. How many bottles make 10 L?
  7. Kim says, "A litre of popcorn weighs 1 kg, like water." Is she right?
  8. On a jug, there are 4 equal spaces between the 0 L mark and the 1 L mark. The water is at the 2nd small mark. How much water is there?
Answers
  1. Neither: they weigh the same, 1 kg each. The feathers take up much more room, but a kilogram is a kilogram.
  2. Halfway between 4 and 5: 4 and a half kilograms.
  3. 1 L of water is about 1 kg, so 3 L is about 3 kg.
  4. Bath water: litres (how much it holds). A cat: kilograms (how heavy).
  5. 2 L take away half a litre is 1 and a half litres.
  6. Count by 2s: 2, 4, 6, 8, 10. That is 5 bottles.
  7. No. The litre-kilogram trick works for water (and drinks like milk and juice). Popcorn is mostly air, so a litre of popcorn weighs much less than a kilogram.
  8. 4 equal spaces make 1 litre, so each space is a quarter of a litre. The 2nd mark is 2 quarters, which is half a litre.

Common mistake

Mixing up "how heavy" and "how much it holds". Kilograms are for heaviness; litres are for capacity. Only for water (and drinks) does 1 L weigh about 1 kg.

Reading a scale without counting the spaces. A small mark between 1 and 2 is not always a half. If there are 4 spaces, each is a quarter. If there are 10 spaces, each is a tenth. Count first.

Where it leads