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There are two systems used for measuring quantities - metric and imperial.

The metric system uses three main units for measuring:

• length in metres (m)
• mass in kilograms (kg)
• volume in cubic metres (m³)

The imperial system uses the following units:

• length in inches, feet and yards
• mass in pounds (lb), ounces (oz) and stones
• volume in gallons

Converting metric units of length

The most common metric unit conversions for length are:

• 1 km = 1,000 m
• 1 m = 100 cm
• 1 cm = 10 mm
• 1 m = 1,000 mm

These can be used to convert the units used to measure length.

Example

The sunflower is 214 cm tall. Calculate the height of the sunflower in metres and millimetres.

There are 100 centimetres in a metre.

There are 10 millimetres in a centimetre.

It is not always easy to decide whether to divide or multiply by the conversion factor.

Metres are larger than centimetres so the number of centimetres will be larger than the number of metres. Divide the number of centimetres by the conversion factor.

Millimetres are smaller than centimetres so the number of millimetres will be larger than the number of centimetres. Multiply the number of centimetres by the conversion factor.

Check the answer - should it be smaller or larger than the original number?

Converting imperial units of length

Units of length can be converted between imperial and metric units using unit conversions. If you need to use an imperial to metric conversion in an exam, the values will be given to you.

Two common conversions are:

• 5 miles ≈ 8 kilometres
• 1 inch ≈ 2.5 centimetres

≈ means 'approximately equal to'.

Example

Convert 250 kilometres into miles.

5 miles ≈ 8 km

1 mile ≈ 1.6 km