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Succeed with maths: part 2
Succeed with maths: part 2

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2.3 Relating SI and imperial: mass

For the units covered in the course, the two systems are related as shown below:


  • 1 kg = 2.2 lb
  • 1 ounce = 28.3 g
  • 1 stone = 6.4 kg

If you are more familiar with imperial measurements, this will hopefully show how small a gram actually is. To get a feel for this, a paperclip has a mass of around a gram – so not very much in everyday terms!

To give yourself a good feel for both systems of measurement, try weighing a few everyday objects around you. Note down the mass in both systems of measurement.

Hopefully, you will now feel confident enough to convert between the SI and imperial system for mass without an example. So, it’s straight into our next activity. Remember to click on ‘reveal comment’ if you need a hint.

Activity 4 Relating SI and imperial units of mass

Timing: Allow approximately 5 minutes
  • a.A baby weighs 3.7 kg when it is born. How much is this in pounds and ounces?


one kg equals 2.2 lb

This means converting from a physically larger unit to a smaller one. The final answer should therefore be larger.


  • kg equals 2.2 lb

    multiline equation line 1 Mass of baby in lb equals 3.7 multiplication 2.2 lb line 2 equals 8.14 lb

    So the mass is 8 lb and 0.14 lb

    one lb equals 16 oz

    multiline equation line 1 0.14 lb in ounces equals 0.14 multiplication 16 oz line 2 equals 2.24 oz

    So, the baby weighed 8 lb and 2 oz (to the nearest ounce) when it was born.

  • b.One litre of water has a mass of 1 kg. If a rainwater butt contains 120 litres of water, what mass of water does it contain?


b. one kg equals 2.2 lb

multiline equation line 1 Mass of water in kg equals 120 multiplication one kg equals 120 kg

multiline equation line 1 Mass of water in lb equals 120 multiplication 2.2 lb equals 264 lb

Now, onto our next section, which is about how the capacity of something is measured – that is, its volume. This is another of those measurements that is very familiar in everyday life, whether it is the quantity of milk in a container or how much fuel is needed to fill a car.