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1 millimetre =
SI units
1×10−3 m 0.1 cm
US customary / Imperial units
3.2808×10^−3 ft 39.370×10^−3 in

The millimetre (International spelling as used by the International Bureau of Weights and Measures) or millimeter (American spelling) (SI unit symbol mm) 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 1,000 micrometres and 1,000,000 nanometres. There are 25.4 mm in one inch by definition, so a millimetre is exactly equal to 5127 inch.


Since 1983, the metre has been defined as "the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second."1 A millimetre, 1/1000 of a metre, is therefore the distance travelled by light in one 1/299,792,458,000 of a second.

Unicode symbols

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

  • millimetre (㎜) - code U+339C2
  • square millimetre (㎟) - code U+339F2
  • cubic millimetre (㎣) - code U+33A32


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 0.01mm.4

Microwaves with a frequency of 300 GHz have a wavelength of 1 mm. Using wavelengths between 30 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 and 0.18 mm thick, with ordinary printer paper or copy paper approximately a tenth of a millimetre thick.7 The thinnest phone as of 2013 is the Huawei Ascend P6, at 6.2 mm thick.8

See also


  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". 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. 
  8. ^ "Huawei Ascend P6 review". Tech Radar. Retrieved 3 December 2013. 

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