Converting between metric and imperial units
Maths · Key Stage 2 · The School
Two systems, side by side
Britain mostly uses metric units: metres, kilograms and litres. But some imperial units are still common too: miles, pounds and pints. Road signs show miles. Bathroom scales often show pounds. Knowing roughly how the two link up is a useful skill. Older relatives often still think in imperial units, out of habit. Knowing both keeps a chat, or a recipe, from going wrong.
A handful of rough conversions
About 8 kilometres make 5 miles. About 1 kilogram is a little over 2 pounds. About 1 litre is a little under 2 pints. These are rough, not exact. But they are close enough for daily use. They are also far easier to recall than the exact decimal figures. A few rough numbers like these will cover almost any case you meet.
Why "rough" is the right word here
A metric measurement converts to another metric unit exactly. That is because the whole system is built on tens. Metric to imperial is not like that. The two systems grew up apart, centuries apart. Their true conversions are messy decimals. Those decimals just get rounded off for daily use.
Using a rough conversion
An old recipe lists a weight in pounds: 4 pounds of flour. Use the rough rule that 1 kilogram is a bit over 2 pounds. Divide 4 by roughly 2. That gives about 2 kilograms. It will not be exactly right. But it is close enough to weigh out the flour well.