Millimeter (mm)
Definition
A millimeter (symbol mm) is a metric unit of length equal to one thousandth of a meter, or exactly 1,000 micrometers. It is the standard unit for precision engineering drawings, machining tolerances, rainfall measurement, and small medical dimensions.
History
The millimeter dates to the introduction of the metric system in France in 1795, formed by combining the SI prefix milli (Latin mille, thousand) with the meter. It quickly became the working unit of mechanical engineering across Europe.
ISO standards now specify millimeter as the default dimension unit on technical drawings, and most countries use it for rainfall and precipitation reporting. The 2019 SI revision did not change its magnitude; it remains exactly one thousandth of a meter.
Standard reference
Defined by the BIPM SI Brochure as 0.001 × the meter. ISO 128 specifies millimeter as the default dimension unit on technical drawings.
Common conversions
| 1 mm | = 0.001 m |
|---|---|
| = 0.1 cm | |
| = 0.0393701 in | |
| = 1,000 μm |