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 millimetre and centimetre marks
General information
Unit systemSI derived unit
Unit ofLength
Named afterThe metric prefix mille (Latin for "one thousand") and the metre
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. Since an inch is officially defined as exactly 25.4 millimetres, a millimetre is therfore equal to exactly 5127 (≈ 0.03937) 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.

Informal terminology

A common shortening of millimetre in spoken English is "mil". This can cause confusion since in the United States, "mil" traditionally means a thousandth of an inch.

Unicode symbols

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 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 also


  1. "17th General Conference on Weights and Measures (1983), Resolution 1". International Bureau of Weights and Measures. Retrieved 3 December 2013.
  2. "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". Archived from the original on 8 June 2017. Retrieved 3 December 2013.
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