Converting between metric units
Maths · Key Stage 2 · The School
Metric units all link by tens
The metric system is built on tens, hundreds and thousands. That makes converting between units simple multiplying or dividing. 10 millimetres make 1 centimetre. 100 centimetres make 1 metre. 1,000 metres make 1 kilometre. Every step is a clean multiple of ten. Nothing messy or awkward hides in a metric conversion. That is exactly why the system was designed this way.
Going from big units to small
To go from a bigger unit to a smaller one, multiply. There are more small units in the same amount, so the number gets bigger. 3 kilograms is 3,000 grams, because you multiply by 1,000. 2 litres is 2,000 millilitres, for the same reason. A bigger number for a smaller unit always makes sense, once you picture it: it simply takes more of the small unit to make the same amount.
Going from small units to big
To go from a smaller unit to a bigger one, divide instead. There are fewer big units in the same amount, so the number gets smaller. 4,500 grams is 4.5 kilograms, because you divide by 1,000. 250 centimetres is 2.5 metres, because you divide by 100. Getting the direction right matters more than the arithmetic itself. Ask which unit is bigger before you decide whether to multiply or divide.
Converting a mixed measurement
A recipe needs 1 kilogram and 250 grams of flour, but the scales only show grams. Convert the kilogram first: 1 kilogram is 1,000 grams. Add the extra 250 grams. The total is 1,250 grams. Converting to one single unit before adding is the safest way to avoid a mistake.