Millimetre

The millimetre (symbol mm) is a unit of length in the International System of Units (SI). One millimetre equals one thousandth of a metre, or 0.001 metre. Consequently, one metre contains 1,000 millimetres.

The term derives from the prefix 'milli-', from Latin denoting one thousandth, combined with 'metre'. The millimetre is widely used in small-scale measurements in engineering, construction, health, and science, because the metre alone is too large for measuring small objects.

Relationship to other units

One centimetre contains 10 millimetres. One millimetre contains 1,000 micrometres (µm), and each micrometre contains 1,000 nanometres. In the Imperial system, one inch is defined as exactly 25.4 millimetres; this is an agreed formal definition.

Uses

Measuring instruments such as rulers and measuring tapes are typically marked in millimetres. In mechanical engineering, the dimensions of machine parts are usually expressed in millimetres, as are blueprint drawings. In meteorology, rainfall is measured in millimetres, with one millimetre equivalent to one litre of water falling on one square metre of ground.

Symbol and notation

The official symbol is 'mm', always written in lowercase, and does not take an 's' for plural. When accompanied by a number, a space typically separates the number and symbol, as in '15 mm'.

Related units and conversions

The square millimetre (mm²) is a unit of area, while the cubic millimetre (mm³) is a unit of volume equivalent to one microlitre. There is also 'millimetre of mercury' (mmHg), a unit of pressure still used in medicine, particularly for measuring blood pressure.