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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.

Ruler with millimeter and centimeter marks.png
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    1×103 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

One millimetre is equal to 1000 micrometres or 1000000 nanometres. A millimetre is equal to exactly ​5127 (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".[1] A millimetre, 1/1000 of a metre, is therefore the distance travelled by light in 1/299792458000 of a second.

Unicode symbolsEdit

For the purposes of compatibility with Chinese, Japanese and Korean (CJK) characters, Unicode has symbols for:

  • millimetre (㎜) - code U+339C[2]
  • square millimetre (㎟) - code U+339F[2]
  • cubic millimetre (㎣) - code U+33A3[2]

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] High-quality 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]

See alsoEdit


  1. ^ "17th General Conference on Weights and Measures (1983), Resolution 1". International Bureau of Weights and Measures. Retrieved 3 December 2013. 
  2. ^ a b c "CJK Compatibility" (PDF). Retrieved 3 December 2013. 
  3. ^ "How do I read a ruler?". Retrieved 3 December 2013. 
  4. ^ "Accuracy of Calipers". Retrieved 3 December 2013. 
  5. ^ Huang, Kao-Cheng; Wang, Zhaocheng (2011). Millimeter Wave Communication Systems. ISBN 9781118102756. 
  6. ^ "How Small Can the Naked Eye See?". Focus Magazine. Retrieved 3 December 2013. 
  7. ^ "Thickness of a Piece of Paper". Retrieved 3 December 2013.