Millimeter

A millimeter (symbol: mm) is a unit of length in the International System of Units (SI). One millimeter equals one-thousandth of a meter, or 0.001 m. One centimeter contains ten millimeters, and one meter contains one thousand millimeters.

The term combines the prefix 'milli-', meaning one-thousandth, with 'meter'. Millimeters are commonly used for small measurements in engineering, construction, and science, as they provide precise values without requiring extensive decimals.

Definition and relationship to other units

The meter is the fundamental SI unit of length, defined by the speed of light in vacuum. The millimeter is derived from the meter: 1 mm = 10⁻³ m. Related units include the micrometer (1 mm = 1,000 µm) and the nanometer (1 mm = 1,000,000 nm). In the imperial system, one inch equals exactly 25.4 mm.

Applications

In technical drawing and manufacturing, measurements are typically recorded in millimeters. Instruments such as calipers and micrometers measure in millimeters and their subdivisions. Rainfall is measured in millimeters, indicating the depth of water accumulated on a level surface; one millimeter of rain is equivalent to one liter of water spread over one square meter.

Notation and symbols

The correct symbol is 'mm', always written in lowercase. When appearing within a sentence, it is not followed by a period. In some contexts, particularly in medicine, 'mmHg' (millimeters of mercury) is used as a unit of pressure, for example in measuring blood pressure.