Kilometres and millimetres · Usually met around P3
Millimetres
1 cm = 10 mm.
How to do it
Method
1 cm = 10 mm
4 cm 7 mm = 4 × 10 + 7 = 47 mm
Another way
Count the small steps
Count 47 small steps from 0
In full
A millimetre (mm) is shorter than a centimetre: each centimetre on a ruler is cut into 10 small steps, each 1 mm, so 1 cm = 10 mm. A coin's thickness or an ant's length is measured in millimetres. Line one end up with 0, read the whole centimetres, then count the small steps: 4 cm and 7 steps is 4 cm 7 mm, which is 4 × 10 + 7 = 47 mm. If an end isn't at 0, take its reading away from the other end's. Or count every small step from 0.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
A coin is about 3 □ thick. Which unit goes in □?
- A. m
- B. mm
- C. cm
- D. km
Choose the right answer
Show the answer and working
Answer: B. mm
A coin is very thin, so use the smallest unit, millimetres: about 3 mm.
Example 2
3 cm 5 mm = □ mm
Show the answer and working
Answer: 35
3 × 10 + 5 = 35.
Example 3
178 mm = □ cm □ mm
Fill in each box
Show the answer and working
Answer: 17，8
Every 10 mm is 1 cm: 178 mm is 17 lots of 10 mm and 8 mm more.
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