# Millimetre

The millimetre (international spelling; SI unit symbol mm) or millimeter (American spelling) is a unit of length in the International System of Units (SI), equal to one thousandth of a metre, which is the SI base unit of length. Therefore, there are one thousand millimetres in a metre. There are ten millimetres in a centimetre.

millimetre Ruler with millimetre and centimetre marks
General information
Unit systemSI
Unit ofLength
Symbolmm
Named afterThe metric prefix mille (Latin for "one thousand") and the metre
Conversions
1 mm in ...... is equal to ...
micrometres   1×103 μm = 1000 μm
centimetres   1×10−1 cm = 0.1 cm
metres   1×10−3 m = 0.001 m
kilometres   1×10−6 km
inches   0.039370 in
feet   0.0032808 ft Different lengths as in respect to the electromagnetic spectrum, measured by the metre and its derived scales. The microwave is between 1 meter to 1 millimeter.

One millimetre is equal to 1000 micrometres or 1000000 nanometres. Since an inch is officially defined as exactly 25.4 millimetres, a millimetre is equal to exactly 5127 (≈ 0.03937) of an inch.