a 6 μm diameter carbon filament,
compared to 50 μm diameter human hair
Unit information
Unit system metric
Unit of length
Symbol μm
Unit conversions
1 μm in ...... is equal to ...
   SI units    1×10−6 m
   Natural units    6.1877×1028 P
1.8897×104 a0
   imperial/US units    3.2808×10−6 ft
3.9370×10−5 in
"Micron" redirects here. For other uses, see Micron (disambiguation).
"Microscale" redirects here. For other uses, see Microscale (disambiguation).
For the measuring instrument, see Micrometer.

The micrometre (International spelling as used by the International Bureau of Weights and Measures;[1] SI symbol: μm) or micrometer (American spelling), also commonly known as a micron, is an SI derived unit of length equaling 1×10−6 of a metre (SI standard prefix "micro-" = 10−6); that is, one millionth of a metre (or one thousandth of a millimetre, 0.001 mm, or about 0.000039 inch).[1] The symbol μm is sometimes rendered as um if the symbol μ cannot be used, or if the writer is not aware of the distinction.

The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of biological cells and bacteria and is also commonly used in plastics manufacturing.[1] Micrometres are the standard for grading wool by the diameter of the fibres; wool finer than 25 μm can be used for garments, while coarser grades are used for outerwear, rugs, and carpets.[2] The width of a single human hair ranges from approximately 10 to 200 μm. The first and longest human chromosome is 10μm in length.


Between 1 μm and 10 μm:

For more examples of things measuring between one and ten micrometres, see 1 micrometre.

Between 10 μm and 100 μm

For more examples of things measuring between ten and 100 micrometres, see 10 micrometres.

SI standardization

The term micron and the symbol μ were officially accepted for use in isolation to denote the micrometre in 1879, but officially revoked by the International System of Units (SI) in 1967.[7] This was necessary because that older usage became incompatible with the official adoption of the unit prefix micro-, denoted μ, during the creation of the SI in 1960. In the SI, the systematic name micrometre became the official name of the unit, and μm became the official unit symbol.

Nevertheless, in practice, "micron" remains a widely used term in preference to "micrometre" in many English-speaking countries, both in academic science (including geology, biology, physics, and astronomy) and in applied science and industry (including machining, the semiconductor industry, and plastics manufacturing). Additionally, in American English the use of "micron" helps differentiate the unit from the micrometer, a measuring device, because the unit's name in mainstream American spelling is a homograph of the device's name. In spoken English, they may be distinguished by pronunciation, as the name of the measuring device is invariably stressed on the second syllable, whereas the systematic pronunciation of the unit name, in accordance with the convention for pronouncing SI units in English, places the stress on the first syllable.

The plural of micron is normally "microns", though "micra" was occasionally used before 1950.[8][9][10]


The official symbol for the SI prefix micro- is a Greek lowercase mu (μ).[11] In Unicode, there is also a micro sign with the codepoint U+00B5 (µ), distinct from the codepoint U+03BC (μ) of the Greek letter lowercase mu, so that machines can recognize it as the SI prefix symbol rather than as a letter. According to The Unicode Consortium, the Greek letter character is preferred,[12] but implementations must recognize the micro sign as well. Most fonts use the same glyph for the two characters.

See also

Look up micrometre in Wiktionary, the free dictionary.

Notes and references

  1. 1 2 3 "micrometre". Encyclopedia Britannica Online. Retrieved 18 May 2014.
  2. "Wool Fibre". "NSW Department of Education and Communities". Retrieved 18 May 2014.
  3. Smith, D.J. (2009). "Human sperm accumulation near surfaces: a simulation study" (PDF). Journal of Fluid Mechanics. 621: 295. doi:10.1017/S0022112008004953. Retrieved 20 May 2012.
  4. Gordon Ramel. "Spider Silk". Archived from the original on 4 December 2008. Retrieved 2008-12-04. garden spider silk has a diameter of about 0.003 mm ... Dragline silk (about .00032 inch (.008 mm) in Nephila)
  5. IST - Innovative Sintering Technologies Ltd. "Fibreshape applications". Retrieved 2008-12-04. Histogram of cotton thickness
  6. According to The Physics Factbook, the diameter of human hair ranges from 17 to 181 μm. Ley, Brian (1999). "Width of a Human Hair". The Physics Factbook.
  7. BIPM - Resolution 7 of the 13th CGPM (1967/68), "Abrogation of earlier decisions (micron, new candle.)"
  8. "Proceedings of the Royal Society of Queensland". google.com.br.
  9. "The Observer". google.com.br.
  10. [10 micra/10 microns https://books.google.com/ngrams/graph?content=10+micra%2F10+microns&year_start=1885&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2C%2810%20micra%20/%2010%20microns%29%3B%2Cc0]
  11. Prefixes of the International System of Units, International Bureau of Weights and Measures (page visited on 9 May 2016).
  12. Unicode Technical Report #25
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