Units of Length, Mass and Capacity

Three things we measure

In maths we often need to describe the size of an object. There are three common things we measure, and each one answers a different question.

• Length answers how long, tall or far? — the height of a door, the distance to school.
• Mass answers how heavy? — the mass of a banana or a school bag.
• Capacity answers how much does it hold? — the amount of juice in a carton.

Each of these uses its own set of units. A unit is just an agreed amount we count in, like centimetres or litres. Using the same units lets everyone in the world understand each other's measurements.

Units of length

Length is measured with these metric units, from smallest to largest:

Unit	Short way	Good for measuring
millimetre	mm	the thickness of a coin
centimetre	cm	the width of your hand
metre	m	the height of a room
kilometre	km	the distance between two towns

The units fit together in neat steps:

• 10 mm = 1 cm
• 100 cm = 1 m
• 1000 m = 1 km

So a pencil might be 15 cm long, a classroom 8 m wide, and a walk to the park 2 km.

Units of mass

Mass tells us how heavy something is. The two everyday units are:

• gram (g) — light things, like a strawberry or a paperclip.
• kilogram (kg) — heavier things, like a bag of flour or a cat.

The link between them is simple:

1000 g = 1 kg

A loaf of bread might have a mass of 800 g, while a watermelon could be 3 kg.

Units of capacity

Capacity is how much liquid a container can hold. We use:

• millilitre (ml) — small amounts, like a spoonful of medicine.
• litre (l) — larger amounts, like a bottle of water.

And once again:

1000 ml = 1 litre

A teaspoon holds about 5 ml, a fizzy drink can holds about 330 ml, and a large milk bottle holds 2 litres.

Why metric units jump in tens

You might wonder why these units go up in 10s, 100s and 1000s. The metric system was built to match our place value system, which is also based on ten. Because of this, converting is never about awkward numbers — you only ever multiply or divide by 10, 100 or 1000, just by moving digits. This is the same skill you use in Place Value: Tens and Ones.

Changing between units

Here is the golden rule:

• Going from a big unit to a smaller unit → multiply (you need more of the small units).
• Going from a small unit to a bigger unit → divide (you need fewer of the big units).

Worked example 1: metres to centimetres

Change 4 metres into centimetres.

1. A metre is bigger than a centimetre, so we multiply.
2. There are 100 cm in 1 m.
3. 4 × 100 = 400 cm.

Worked example 2: grams to kilograms

Change 3500 g into kilograms.

1. A gram is smaller than a kilogram, so we divide.
2. There are 1000 g in 1 kg.
3. 3500 ÷ 1000 = 3.5 kg.

Worked example 3: a real problem

A jug holds 1500 ml of water. You pour out 600 ml. How much is left, in litres?

1. Left in the jug: 1500 − 600 = 900 ml.
2. Change to litres by dividing by 1000: 900 ÷ 1000 = 0.9 litres.

Choosing a sensible unit

A big part of measuring well is picking the right unit so your number is not silly. You could say a door is 2000 mm tall, but 2 m is much easier to read. Match the unit to the size of the object.

Object	Sensible unit
Length of an ant	mm
Height of a child	cm or m
Mass of a feather	g
Mass of a person	kg
Medicine in a spoon	ml
Water in a bucket	litres

Try it yourself

Go on a measuring hunt around your home:

1. Find three lengths and write them in cm or m.
2. Find two things to weigh and estimate their mass in g or kg.
3. Find two containers and guess their capacity in ml or litres.

Then check a few with a ruler, kitchen scales or a measuring jug. How close were your estimates?

Great work!

You now know what length, mass and capacity mean, the units we use for each, and how to convert between them. Next, practise actually using a ruler and scales in Reading Scales and Measuring, or build your measuring skills in Measuring Length and Height.