The millimetre (SI symbol: mm; international spelling) or millimeter (American spelling) is a unit of length in the International System of Units (SI), equal to one thousandth of a metre, the SI base unit of length.

- 1 metre = 1000 millimetres - 1 centimetre = 10 millimetres

One millimetre is also equal to: - 1000 micrometres - 1000000 nanometres

Since an inch is officially defined as exactly 25.4 millimetres, 1 millimetre is precisely 5⁄127 inches (≈ 0.03937 inches).

Since 1983, the metre has been defined as "the length of the path travelled by light in vacuum during a time interval of ⁠1/299792458⁠ of a second".

A millimetre, being ⁠1/1000⁠ of a metre, is the distance light travels in ⁠1/299 792 458 000⁠ of a second.

The term "mil" is sometimes used colloquially for millimetre. However, in the United States, "mil" traditionally means a thousandth of an inch, which may cause confusion.

To support layout compatibility with East Asian scripts (CJK), Unicode includes square symbols for: