The millimetre (International spelling as used by the International Bureau of Weights and Measures) or millimeter (American spelling) (SI unit symbol mm) is a unit of length in the metric system, equal to one thousandth of a metre, which is the SI base unit of length. It is equal to 1,000 micrometres and 1,000,000 nanometres. There are 25.4 mm in one inch by definition, so a millimetre is exactly equal to inch.

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