Converting metric units

The metric system is built on tens, which makes converting much easier than with older systems of measurement. The prefix in each unit's name tells you how it relates to the base unit (metre, gram or litre):

• kilo- means 1000: 1 km = 1000 m and 1 kg = 1000 g.
• centi- means one hundredth: 100 cm = 1 m.
• milli- means one thousandth: 1000 mm = 1 m and 1000 mL = 1 L.

There is also a link between millimetres and centimetres: 10 mm = 1 cm. Look at a ruler and you will see 10 small marks in every centimetre.

When you convert from a larger unit to a smaller unit, you need more of the smaller units, so you multiply. For example, 3.5 m in centimetres is 3.5 × 100 = 350 cm.

When you convert from a smaller unit to a larger unit, you need fewer of the larger units, so you divide. For example, 2750 g in kilograms is 2750 ÷ 1000 = 2.75 kg.

Multiplying by 10, 100 or 1000 moves every digit 1, 2 or 3 places to the left. Dividing moves every digit 1, 2 or 3 places to the right. A place value chart makes this easy to see: 2750 ÷ 1000 moves the 2 from the thousands place to the ones place, giving 2.75.

Always check that your answer makes sense. A length in millimetres should be a bigger number than the same length in metres. If you convert 4 km and get 0.004 m, you have divided instead of multiplying.