Millimetres
| Millimetre | |
|---|---|
| Unit system | metric |
| Unit of | length |
| Symbol | mm |
| Unit conversions | |
| 1 mm in ... | ... is equal to ... |
| SI units | 1×10−3 m |
| imperial/US units |
0.0032808 ft 0.039370 in |
The millimetre (International spelling as used by the International Bureau of Weights and Measures; SI unit symbol mm) or millimeter (American spelling) 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 1000 micrometres or 1000000 nanometres. A millimetre is equal to exactly 5⁄127 (approximately 0.039370) of an inch.
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, 1/1000 of a metre, is therefore the distance travelled by light in 1/299792458000 of a second.
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