Millimetres

Millimetre, abbreviated mm, is a unit of length in the metric system. It is defined as one thousandth of a metre, so 1 mm = 0.001 m. The millimetre is a common unit for measuring small distances in science, engineering and everyday life, where centimetres or metres may be too coarse.

The term millimetre derives from milli- meaning thousandth and metre. It is part of the International System of Units (SI). The metre is the base unit of length, and multiples and submultiples such as millimetres are used with standard prefixes.

One metre equals 1000 millimetres, and one centimetre equals 10 millimetres. A micrometre equals 0.001 millimetres, and a millimetre equals about 0.0393701 inches (since one inch is defined as 25.4 millimetres exactly).

Practical usage: tools with metric scales, such as rulers, calipers and micrometres, typically mark lengths in millimetres. In manufacturing tolerances and quality control, measurements are often given in decimals of a millimetre.

Everyday examples: the thickness of a credit card is about 0.76 mm; a human hair commonly ranges from about 10 to 100 micrometres, which is 0.01 to 0.10 mm.