Convert between metric units of length - mm, cm, m
Theory
\(1\) cm \(= 10\) mm, \(1\) m \(= 100\) cm and \(1\) m \(= 1000\) mm. To change to a smaller unit, multiply (there will be more of them); to change to a larger unit, divide. This Year 6 maths page shows how to convert lengths, including decimals such as \(3.6\) m and \(8.5\) cm, and how to compare lengths written in different units.
A millimetre (mm) is the smallest mark on a ruler. A centimetre (cm) is \(10\) mm. A metre (m) is \(100\) cm, or \(1000\) mm.
Larger unit to smaller unit: multiply. \(7\) m \(= 7 \times 100 = 700\) cm.
Smaller unit to larger unit: divide. \(85\) mm \(= 85 \div 10 = 8.5\) cm.
Multiplying or dividing by \(10\), \(100\) or \(1000\) moves the digits \(1\), \(2\) or \(3\) places. \(3.6\) m \(= 360\) cm; \(2750\) mm \(= 2.75\) m.
To compare lengths, first write them in the same unit.
Length conversions:
| Change | Do this |
|---|---|
| m to cm | \(\times 100\) |
| cm to mm | \(\times 10\) |
| m to mm | \(\times 1000\) |
| mm to cm | \(\div 10\) |
| cm to m | \(\div 100\) |
| mm to m | \(\div 1000\) |
How to convert a length
- Decide: is the new unit smaller (multiply) or larger (divide)?
- Choose \(10\), \(100\) or \(1000\) from the facts.
- Move the digits that many places and write the new unit.
| \(9 \times 10\) | \(=\) | \(90\) |
\(90\) mm.
| \(7 \times 100\) | \(=\) | \(700\) |
| \(3.6 \times 100\) | \(=\) | \(360\) |
\(700\) cm and \(360\) cm.
| \(85 \div 10\) | \(=\) | \(8.5\) |
\(8.5\) cm.
(ii) Write \(0.45\) m in millimetres.
| \(2750 \div 1000\) | \(=\) | \(2.75\) |
| \(0.45 \times 1000\) | \(=\) | \(450\) |
(i) \(2.75\) m
(ii) \(450\) mm
| \(6.8 \times 10\) | \(=\) | \(68\) |
\(68\) mm.
(i) Which is longer?
(ii) By how many millimetres?
| \(14 \times 10\) | \(=\) | \(140\) |
| \(140 - 128\) | \(=\) | \(12\) |
(i) The pen (\(140\) mm)
(ii) \(12\) mm
In millimetres: \(900\), \(870\), \(950\).
\(870\) mm, \(0.9\) m, \(95\) cm.
| \(62 \div 10\) | \(=\) | \(6.2\) |
Centimetres are larger than millimetres, so divide: \(6.2\) cm (see the ruler diagram).
Common pitfalls
Frequently asked questions
How many millimetres are in a centimetre?
\(10\).
How many centimetres are in a metre?
\(100\).
How many millimetres are in a metre?
\(1000\).
How do I change centimetres to metres?
Divide by \(100\): \(360\) cm \(= 3.6\) m.
How do I compare 0.9 m and 870 mm?
Change to the same unit: \(900\) mm is more than \(870\) mm.