3.2 Comparing weights
By law, weights of goods for sale in the UK have to be in metric units: grams and kilograms.
Historically, however, most people used imperial measures of weight: in size order these are ounces, pounds and stones.
- 16 ounces (oz) = 1 pound (lb)
- 14 pounds = 1 stone (st)
You might still come across these weights sometimes.
An ounce is a bit less than 30 g. A pound is a bit less than half a kilogram.
Example: Two weight measurements
You have an old ladder with a label that says it can hold up to 20 stone. You weigh 80 kg. Can you safely use the ladder?
Hint: 1 st = 14 lb
You need to work out roughly what 20 stone is in kilograms. First, you need to find out how much 20 stone is in pounds.
- 20 × 14 = 280 lbs
One pound is equivalent to nearly half a kilogram, so next you need to divide the weight in pounds by 2:
- 280 ÷ 2 = 140
The ladder will take about 140 kg – so you’re safe!
Now try the following activity. Remember to check your answers once you have completed the questions.
Activity 8: Converting weights
- An airline’s weight allowance for a piece of hand luggage is 5 kg. You have weighed your bag on some old bathroom scales and found that it is 7 lbs. Can you take it?
- You are using a recipe your grandmother wrote down. It calls for 4 oz sugar. You only have 150 g left. Do you have enough to make the recipe?
- A pound is a bit less than half a kilogram. To estimate what the weight allowance is in pounds, you need to multiply the amount in kilograms by 2:
- 5 × 2 = 10 lbs
Alternatively, you could estimate how much your bag weighs in kilograms by dividing the amount in pounds by 2:
- 7 ÷ 2 = 3.5 kg
Using either method, your bag doesn’t exceed the weight allowance.
- An ounce is a bit less than 30 g. To estimate how much sugar you need in ounces, you need to multiply the amount in ounces by 30:
- 4 × 30 = 120 g
Alternatively, you could estimate how much sugar you have in ounces by dividing the amount in grams by 30:
- 150 ÷ 30 = 5 oz
Using either method, you have enough sugar for the recipe.
In this section you have learned how to:
- estimate and measure weight
- use metric units of weight
- know the relationship between grams and kilograms
- convert from imperial to metric units of weight.