Converting between Units (WJEC GCSE Maths & Numeracy (Double Award)): Revision Note

Converting units

How do I convert units of lengths?

• For metric units, conversion can be done by simply multiplying or dividing by powers of 10

• You need to know the basic conversions for length

  • 1 cm = 10 mm

  • 1 m = 100 cm

  • 1 km = 1000 m

• You need to decide whether to multiply or divide by the conversion factor

  • Ask, does the number of units increase or decrease?

    • The number of mm is 10 times bigger than the number of cm so mm = cm × 10

• You can do the conversion in stages

  • First convert kilometres into metres then metres into centimetres

How do I convert units of mass?

• For metric units, conversion can be done by simply multiplying or dividing by powers of 10

• You need to know the basic conversions for mass

  • 1 g = 1000 mg

  • 1 kg = 1000 g

  • 1 tonne = 1000 kg

What are metric and imperial units?

• All the units above are known as metric units

  • They are based on metres for length, grams for mass, and litres for volume

  • They are based on powers of 10 (e.g. 1 ml is one thousandth of a litre)

• Imperial units are an older system of measurement, but often still used in daily life

• You may encounter:

  • Miles, feet and inches for length

  • Pounds (lb) and ounces for mass

  • Pints and gallons for volume

How do I convert between metric and imperial units?

• In the exam you will be given the appropriate conversion factors

• E.g. You may be told that 1 gallon is approximately (≈) 4.546 litres

  • To convert from gallons to litres, multiply by 4.546

  • To convert from litres to gallons, divide by 4.546