To convert millimeters to centimeters (mm to cm), multiply by 0.1. 1 millimeter equals 0.1 cm, about 17% of a grain of rice.
| millimeters | centimeters |
|---|---|
| 1 millimeter | 0.1 cm |
| 2 millimeters | 0.2 cm |
| 5 millimeters | 0.5 cm |
| 10 millimeters | 1 cm |
| 25 millimeters | 2.5 cm |
| 50 millimeters | 5 cm |
| 100 millimeters | 10 cm |
| 250 millimeters | 25 cm |
| 500 millimeters | 50 cm |
| 1000 millimeters | 100 cm |
Definition: 1/1000 of a meter - the same "milli" prefix as milliliter or milligram.
History: Like the centimeter, it arrived as part of the same 1790s French decimal system, built specifically to make precise, small-scale measurement simple - no more converting between competing local systems of inches, lines, and points.
Current use: Engineering tolerances, screen and camera specs, and rainfall measurements - which is why weather reports outside the US say "20mm of rain" rather than inches.
Definition: 1/100 of a meter - the "centi" prefix doing the same job it does in "century" or "cent."
History: Introduced alongside the meter itself during the French Revolution's metric overhaul, the centimeter was designed from the start to be a convenient, everyday-sized subdivision - a meter is often too big and a millimeter too small for describing things like a person's height or a book's thickness.
Current use: Height, everyday object sizes, and fabric measurements across most of the world; also the standard unit doctors use internationally for body measurements.
The factor is 0.100000, which is derived rather than defined, so any decimal you see has been rounded somewhere.
For mental arithmetic the factor is close enough to 0.1 to use directly: multiply millimeters by 0.1 and the answer is right to within a rounding error.
Going the other way, one centimeter is 10 mm, which is usually the easier direction to work in. Because the forward factor is small, watch the decimal place rather than the digits: a factor-of-ten slip is the common error, not a rounding one.
To anchor the scale: 10 millimeters is about 1 cm.