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. Therefore, there are one thousand millimetres in a metre. There are ten millimetres in a centimetre.
One millimetre is equal to 1000 micrometres or 1000000 nanometres. A millimetre is equal to exactly ^{5}⁄_{127} (approximately 0.039370) of an inch.
Millimetre  

Ruler with millimetre and centimetre marks


Unit information  
Unit system  SI derived unit 
Unit of  Length 
Symbol  mm 
Named after  The metric prefix mille (Latin for "one thousand") and the metre 
Unit conversions  
1 mm in ...  ... is equal to ... 
micrometres  1×10^{3} 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 
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".^{[1]} A millimetre, 1/1000 of a metre, is therefore the distance travelled by light in 1/299792458000 of a second.
For the purposes of compatibility with Chinese, Japanese and Korean (CJK) characters, Unicode has symbols for:
In Japanese typography, these square symbols were historically used for laying out unit symbols without distorting the grid layout of text characters.
On a metric ruler, the smallest measurements are normally millimetres.^{[3]} Highquality engineering rules may be graduated in increments of 0.5 mm. Digital Vernier callipers are commonly capable of reading increments as small as 0.01 mm.^{[4]}
Microwaves with a frequency of 300 GHz have a wavelength of 1 mm. Using wavelengths between 30 GHz and 300 GHz for data transmission, in contrast to the 300 MHz to 3 GHz normally used in mobile devices, has the potential to allow data transfer rates of 10 gigabits per second.^{[5]}
The smallest distances the human eye can resolve is around 0.02 to 0.04 mm, approximately the width of a human hair.^{[6]} A sheet of paper is typically between 0.07 mm and 0.18 mm thick, with ordinary printer paper or copy paper approximately a tenth of a millimetre thick.^{[7]}
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