How to convert units — a guide with examples

How do you convert units?

Converting units is always the same operation: multiply or divide by the fixed conversion factor between the two units. The main difficulty is remembering which way round — so we start with the principle, then go through the four most common groups of units: length, mass, capacity and time, with examples from everyday life.

The principle for converting units

From a bigger unit to a smaller one: multiply by the conversion factor · From a smaller unit to a bigger one: divide by the conversion factor

Going down the ladder (to a smaller unit) you multiply; coming back up (to a bigger unit) you divide by exactly the same factor.

Worked examples, step by step

Converting kilometres to metres

How many metres are there in 2.5 km?

We are going from a bigger unit (km) to a smaller one (m), so we multiply
The conversion factor between km and m is 1000: 1 km = 1000 m
Multiply: 2.5 × 1000 = 2500
Answer: 2.5 km is 2500 metres

Converting centimetres to metres

How many metres are there in 340 cm?

We are going from a smaller unit (cm) to a bigger one (m), so we divide
The conversion factor between cm and m is 100: 1 m = 100 cm
Divide: 340 ÷ 100 = 3.4
Answer: 340 cm is 3.4 metres

Converting kilograms to grams and comparing masses

Which is heavier: a bag of 1.2 kg, or a bag of 1150 g?

Convert 1.2 kg to grams so that both are in one unit: 1 kg = 1000 g
Multiply: 1.2 × 1000 = 1200
Compare: 1200 g against 1150 g
Answer: the 1.2 kg bag (1200 g) is heavier than the 1150 g bag

Converting an hour and a half to minutes, and finding a speed

A car travels 90 km in an hour and a half. How many minutes is that, and what is its speed in km/h?

Convert an hour and a half to minutes: 1 hour = 60 minutes, so 1.5 × 60 = 90
Speed in km/h is worked out straight from the time in hours, with no need to convert to minutes: 90 km ÷ 1.5 hours
Calculate: 90 ÷ 1.5 = 60
Answer: an hour and a half is 90 minutes, and the speed is 60 km/h

Now try it yourself

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Unit converter

Length, mass, capacity and time — enter a value and get an instant conversion to every unit.

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Learn measurement

Units of length, area and volume and the conversions between them — with practice.

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Printable worksheets

Measurement practice by year group with hints and full solutions.

Frequently asked questions

How do you remember when to multiply and when to divide?

The simple rule: when you move to a smaller unit you need more of them to describe the same amount — so you multiply by the conversion factor. When you move to a bigger unit you need fewer — so you divide. For example km to m (a smaller unit) — multiply; cm to m (a bigger unit) — divide.

How do you convert units of area, such as cm² to m²?

For units of area the factor itself is squared, because there are two dimensions. Since 1 m is 100 cm, the conversion factor between cm² and m² is 100 squared — that is 10 000. So 1 m² is 10 000 cm².

How many cm³ (cubic centimetres) are there in one litre?

1000 cm³. This is a useful fact for the volume of liquids: 1 millilitre is exactly 1 cm³, so 1000 millilitres — which is one litre — is 1000 cm³.

What is the most common mistake in converting length units?

Mixing up the factor 100 with the factor 1000: 1 metre is 100 centimetres, not 1000. The factor 1000 belongs to converting between km and m, not between m and cm. It is always worth checking the full chain of units: km, m, cm, mm. And remember that British road signs use miles: 5 miles is roughly 8 km.

✍️ Written by the MathsUK teamChecked against the National Curriculum and GCSE specificationsLast updated: 13.09.2026