Smaller Than a Metre

How long is your thumbnail? How thick is a coin? "A tiny bit of a metre" is not a useful answer. For small things, we need smaller units.

Centimetres

In the Metre lesson we saw that a metre is about the length of a guitar. Split one metre into 100 equal parts, and each part is a centimetre. Its symbol is cm.

So 1 metre = 100 centimetres.

A centimetre is about the width of your little fingernail. The big numbered marks on most school rulers are centimetres. A pencil might be about 15 cm long, and a sheet of paper for a printer is about 30 cm long.

Millimetres

For even smaller things, split one centimetre into 10 equal parts. Each part is a millimetre. Its symbol is mm.

So 1 centimetre = 10 millimetres, and since a metre has 100 centimetres, 1 metre = 1,000 millimetres.

A bank card is a little less than a millimetre thick. The tiny marks between the numbers on a ruler are millimetres.

Reading a ruler

To measure something with a ruler:

1. Put the end of the thing exactly at the 0 mark (not at the end of the ruler - on many rulers they are not the same).
2. Look at where the other end of the thing reaches.
3. Read the last centimetre number it passed, then count the small millimetre marks after it.

A paper clip that reaches 3 centimetre marks and 2 more small marks is 3 cm and 2 mm long. We can also write that as 3.2 cm, or as 32 mm - all three say the same length.

Try it at home

1. Measure your thumbnail, a coin and a pencil with a ruler, starting at the 0 mark each time.
2. Write each one in centimetres and millimetres, like "3 cm and 2 mm".
3. Which of the three would you rather measure in millimetres only? Why?

Words to know

• centimetre - one hundredth of a metre; its symbol is cm. 100 cm = 1 m
• millimetre - one tenth of a centimetre, one thousandth of a metre; its symbol is mm. 10 mm = 1 cm

Big idea

For small things, a metre is split into 100 centimetres, and each centimetre into 10 millimetres. The big marks on a ruler are centimetres; the tiny ones are millimetres.