The length of an object is how many centimetre spaces it covers on the ruler, not how many lines it touches. When one end sits exactly on the 0 mark, the number at the other end is the length. When the object does not start at 0, take the starting number away from the finishing number. From 2 cm to 8 cm is 8 − 2 = 6 cm, because there are six one-centimetre spaces in between.
Explore this idea with maths tools
Maths you can move
Build 8 + 5
Tap two gold counters to fill the ten-frame.
8 in the frame + 5 waiting. The total stays the same.
Words to know
- length
- how long something is, measured in units such as centimetres
- centimetre
- a unit of length; there are 100 in a metre
- mark
- a numbered line on a ruler
Worked examples
Pencil from 0 cm to 8 cm
- 1.The pencil starts on 0.
- 2.The finishing mark is 8, so it is 8 cm.
= 8
Ribbon from 2 cm to 8 cm
- 1.It does not start at 0.
- 2.8 − 2 = 6 cm.
= 6
Straw from 3 cm to 12 cm
- 1.Take start from end: 12 − 3.
- 2.12 − 3 = 9 cm.
= 9
For parents
Method: Line the object up with 0, then read the far end.
What to say: Ask them to run a finger along the spaces and count those, not the lines.
Common mistakes:
- Counting the lines, which adds one (0 to 6 read as 7).
- Reading the end number when the object does not start at 0.