3
Measuring in cm and mm
Discover Mathematics P3: Term 1, Lessons 1–10
GoalMeasure in centimetres and millimetres, and add lengths laid end to end.
New words
millimetre (mm) · a very small unit of length; 10 mm make 1 cmrecord · write down a measurement with its unittotal length · all the lengths added together
Explanation
Look closely at a ruler. Between two numbers there are 10 small spaces, and each one is 1 millimetre (mm). So 10 mm make 1 cm, and 5 cm is 50 mm. We use mm for very small things, such as an ant, a seed or the thickness of a coin. A length of 3 cm and 4 small marks is 3 cm 4 mm, which is 34 mm. Always write the unit in the table. To find the total length of things laid in a line, add their lengths: 12 cm + 8 cm + 15 cm = 35 cm. Since 1 m is 100 cm, this total is less than 1 m.
Worked examples
A paper clip lies on the ruler. It ends at 4 cm and 7 small marks, so it is 4 cm 7 mm, or 47 mm.
| Object | Length in mm | Length in cm and mm |
|---|---|---|
| Ant | 9 mm | 0 cm 9 mm |
| Bean | 15 mm | 1 cm 5 mm |
| Coin | 24 mm | 2 cm 4 mm |
| Paper clip | 32 mm | 3 cm 2 mm |
Marina lays a pencil of 12 cm, a spoon of 8 cm and a toy car of 15 cm in a line. The total length is 12 + 8 + 15 = 35 cm. That is less than 1 m, because 35 cm is less than 100 cm.
Class activity
Small things hunt. Find 5 small things, such as a bean, a button, a paper clip, a coin and a pin. Measure each one in mm and record it in a table with the unit. Then lay three of them end to end and add their lengths.
Practice
1
How many millimetres long is the line?
28 mm. It is 2 cm and 8 small marks.
2
6 cm = ___ mm.
60 mm, because 1 cm is 10 mm.
3
A seed is 45 mm long. Write its length in cm and mm.
4 cm 5 mm.
4
Nour wants to measure the thickness of a coin. Should she record it in cm or mm? Why?
In mm, because a coin is thinner than 1 cm.
5
Three ribbons are 45 cm, 38 cm and 30 cm long. They are laid end to end. What is the total length? Is it more or less than 1 m?
45 + 38 + 30 = 113 cm. It is more than 1 m (100 cm).