millímetres

Millimetres, written millimetres in British English and abbreviated as mm, are a unit of length in the metric system. A millimetre is defined as one thousandth of a metre, so 1 metre equals 1000 millimetres and 1 centimetre equals 10 millimetres. The prefix milli- denotes 10^-3, making the millimetre a common unit for small to moderate lengths in science, engineering, manufacturing, and everyday measurement.

Historically, the metre was introduced with the metric system in the late 18th century and is now defined in terms of fundamental constants. The millimetre, as a subdivision of the metre, has become a standard unit for precise dimensions and tolerances in technical contexts.

Measuring and specification rely on common instruments such as rulers, calipers, and micrometers. In industry, tolerances are frequently expressed in tenths or hundredths of a millimetre. The millimetre is also widely used for component dimensions in electronics, machinery, and construction.

Common conversions include: 25.4 millimetres equal 1 inch; 10 millimetres equal 1 centimetre; 1000 millimetres equal 1 metre; 1,000,000 millimetres equal 1 kilometre. For perspective, a typical credit card thickness is about 0.76 millimetre, and human hair usually ranges from roughly 50 to 100 micrometres (0.05–0.1 millimetre).

The millimetre remains a fundamental unit in everyday life and technical fields, balancing practicality for manual measurement with compatibility across scientific disciplines.