Look very closely at your own ruler, right between the big number 3 and the big number 4. Can you see the tiny little marks crowded in between them? How many little gaps do you think you could count from one big number to the next?

Watch as we read one length on the ruler together, very carefully. First we find the last whole centimetre the object reaches, then we count on in those little millimetre marks to the very end.

Now we read a length on the board together. We look at the object on the ruler and read it first to the nearest whole centimetre, and then read it more carefully to the nearest millimetre.

Now measure four small classroom items with your own ruler, reading each one to the nearest millimetre: a crayon, a rubber, a key and a lolly stick. Write each one as "___ cm ___ mm".

What we learned

• There are ten millimetres in every centimetre, shown by the little marks between the big numbers.
• To read a length, find the last whole centimetre, then count on in millimetres to the end.
• A length like 4 cm 5 mm can also be written as 45 mm: multiply the centimetres by 10, then add the extra millimetres (4 × 10 + 5 = 45).

Coming up

Next we look at measuring longer things, choosing between centimetres and metres so we pick the unit that fits.