# Micrometre

Short description: Unit of length; one millionth of a metre

micrometre
A 6 μm diameter carbon filament above a 50 μm diameter human hair
General information
Unit systemSI
Unit oflength
Symbolμm
Conversions
1 μm in ...... is equal to ...
SI base units   10−6 m
Natural units   1.8897×104 a0
imperial/US units   3.9370×10−5 in

The micrometre (Commonwealth English) as used by the International Bureau of Weights and Measures;[1] SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron,[2] is a unit of length in the International System of Units (SI) equalling 1×10−6 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 0.00004 inch).[1]

The nearest smaller common SI unit is the nanometre, equivalent to one thousandth of a micrometre, one millionth of a millimetre or one billionth of a metre (0.000000001 m).

The micrometre is a common unit of measurement for wavelengths of infrared radiation as well as sizes of biological cells and bacteria,[1] and for grading wool by the diameter of the fibres.[3] The width of a single human hair ranges from approximately 20 to 200 μm.

## Examples

How big is 1 micrometre?

Between 1 μm and 10 μm:

Between 10 μm and 100 μm:

• about 10–12 μm – thickness of plastic wrap (cling wrap)
• 10 to 55 μm – width of wool fibre[6]
• 17 to 181 μm – diameter of human hair[7]
• 70 to 180 μm – thickness of paper

## 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.[8] This became necessary because the older usage was 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.

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 often stressed on the second syllable (/mˈkrɒmɪtər/ my-KROM-it-ər), 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 (/ˈmkrmtər/ MY-kroh-meet-ər).

The plural of micron is normally microns, though micra was occasionally used before 1950.[9][10][11]

## Symbol

The official symbol for the SI prefix micro- is a Greek lowercase mu.[12] In Unicode, there is also a micro sign with the code point U+00B5 (µ), distinct from the code point U+03BC (μ) of the Greek letter lowercase mu. According to the Unicode Consortium, the Greek letter character is preferred,[13] but implementations must recognize the micro sign as well. Most fonts use the same glyph for the two characters.

## Notes and references

1. "micrometre". Encyclopædia Britannica Online. Retrieved 18 May 2014.
2. "Writing with SI (Metric System) Units". Nist. 13 January 2010.
3. "Wool Fibre" (Word Document download). NSW Department of Education and Communities.
4. Ramel, Gordon. "Spider Silk". "A typical strand of garden spider silk has a diameter of about 0.003 mm ... Dragline silk (about .00032 inch (.008 mm) in Nephila)"
5. Smith, D.J.; Gaffney, E.A.; Blake, J.R.; Kirkman-Brown, J.C. (25 February 2009). "Human sperm accumulation near surfaces: a simulation study". Journal of Fluid Mechanics (Cambridge University Press) 621: 295. doi:10.1017/S0022112008004953. Bibcode2009JFM...621..289S.
6. "Fibreshape applications". IST - Innovative Sintering Technologies Ltd.. "Histogram of Fiber Thickness [micrometre]"
7. The diameter of human hair ranges from 17 to 181 μm. Ley, Brian (1999). "Diameter of a human hair". in Elert, Glenn. Retrieved 2018-12-08.
8. BIPM - Resolution 7 of the 13th CGPM 1967/68), "Abrogation of earlier decisions (micron, new candle.)"
9. Proceedings of the Royal Society of Queensland. Part I. XIX. H. Pole & Co.. 1907.
10. Bigalow, Edward Fuller; Agassiz Association (1905). The Observer. 7–8.
11. 10 micra/10 microns (Start at 1885; before that, the word "micron", singular or plural, was rare)
12. Beeton, Barbara; Freytag, Asmus; Sargent, Murray III (30 May 2017). "Unicode Technical Report #25". Unicode Consortium. p. 11.