Units of Measurement

1 metre is 100 centimetres. 1 kilogram is 1,000 grams. 1 litre is 1,000 millilitres.

Which way to go

Going to a smaller unit means multiplying, because it takes more of them.

Time is different

1 hour is 60 minutes, and 1 minute is 60 seconds. Time does not count in tens.

A check that works

Smaller unit, bigger number. If your answer got smaller, you went the wrong way.

The relationships worth knowing

In metric: 1 km = 1,000 m, 1 m = 100 cm, 1 kg = 1,000 g, 1 L = 1,000 mL. Every relationship is a power of ten, which is why metric conversion is multiplying or dividing by 10, 100 or 1,000 — and never anything awkward.

Which way to multiply

Converting to a smaller unit gives a bigger number, so you multiply. Converting to a bigger unit gives a smaller number, so you divide. 3 km is 3,000 m; 3,000 g is 3 kg. Decide the direction before touching the arithmetic.

Recording conversions in a table

A two-column table with metres in one column and centimetres in the other makes a conversion into a pattern you can extend: 1 and 100, 2 and 200, 3 and 300. Seeing it as a times table rather than a rule makes it much harder to get backwards.

Customary units are less tidy

12 inches to a foot, 3 feet to a yard, 16 ounces to a pound. These are not powers of ten, so each must be learned separately. That awkwardness is precisely why scientists everywhere use metric.