Divide by 1,000 — and it's exact
Milli- means one thousandth, so 1 mm = 0.001 m by definition. There is no approximation and no rounding: 1,000 is a power of ten, so converting just moves the decimal point three places left. To go the other way, multiply metres by 1,000.
Common values
| Millimetres | Metres |
|---|---|
| 1 mm | 0.001 m |
| 10 mm | 0.01 m |
| 15 mm | 0.015 m |
| 100 mm | 0.1 m |
| 500 mm | 0.5 m |
| 750 mm | 0.75 m |
| 1000 mm | 1 m |
| 2500 mm | 2.5 m |
| 5000 mm | 5 m |
The same shift works for decimals: 750.5 mm is 0.7505 m, and a 0.5 mm machining tolerance is 0.0005 m.
How mm and m sit among the metric prefixes
- Centimetre. 1 cm = 10 mm = 0.01 m, so 100 mm is 10 cm.
- Micrometre. 1 mm = 1,000 µm, so metres to micrometres is a factor of a million.
- Why mm on drawings. Engineers work in millimetres to avoid decimals: "25 mm" is clearer and less error-prone than "0.025 m," so conversion to metres is usually only for documentation.
Common questions
How many millimetres are in a metre?
Exactly 1,000. The prefix milli- means one thousandth, so a millimetre is one thousandth of a metre and a metre is 1,000 millimetres.
How do I convert mm to metres?
Divide by 1,000, or move the decimal point three places to the left. 2,500 mm is 2.5 m; 750 mm is 0.75 m; 15 mm is 0.015 m.
Is 1 mm = 0.001 m exact?
Yes, completely exact. It is a definition, not a rounded figure, so there is no error at any number of decimal places. This holds identically in every country that uses SI units.
