View source for Micrometre

{{Short description|Unit of length; one millionth of a metre}}
{{hatnote group|
{{for|the measuring instrument|Micrometer (device)}}
{{Redirect-multi|2|Micron|Microscale}}
{{redirect|μm|the chemical unit "μM"|Micromolar}}
}}
{{Use Oxford spelling|date=September 2019}}
{{Use dmy dates|date=February 2021}}
{{Infobox unit
| image    = [[File:Cfaser haarrp.jpg|300px]]
| caption  = A 6&nbsp;μm diameter [[Carbon fibers|carbon filament]] above a 50&nbsp;μm diameter human hair
| name     = micrometre
| symbol   = μm
| standard = [[SI]]
| quantity = [[length]]
| units1   = [[SI base units]]
| inunits1 = {{val|e=-6|ul=m}}
| units2   = [[Natural units]]
| inunits2 = {{val|1.8897|e=4|u=[[Bohr radius|''a''<sub>0</sub>]]}}
| units3   = [[imperial units|imperial]]/[[US customary units|US]]&nbsp;units
| inunits3 = {{convert|1|um|in|disp=out|lk=on|sigfig=5}}
}}

The '''micrometre''' ([[English in the Commonwealth of Nations|Commonwealth English]]) or '''micrometer''' ([[American English]]) ([[SI]] symbol: '''μm''') is a unit of [[length]] in the [[International System of Units]] (SI) equalling {{val|e=-6|u=[[metre]]}} (SI standard prefix "[[micro-]]" = {{val|e=-6}}); that is, one millionth of a [[metre]] (or one thousandth of a [[millimetre]], {{val|0.001|u=mm}}, or about {{val|0.00004|u=[[inch]]}}).<ref name="Encyclopædia Britannica Online">{{cite encyclopedia|url=https://www.britannica.com/science/International-System-of-Units|title=micrometre|encyclopedia=[[Encyclopædia Britannica Online]]|access-date=18 May 2014}}</ref>

The nearest smaller common [[SI Unit|SI unit]] is the [[nanometre]], equivalent to one thousandth of a micrometre, one millionth of a millimetre or one billionth of a metre ({{val|0.000000001|u=m}}).

The micrometre is a common unit of [[measurement]] for [[wavelength]]s of [[infrared radiation]] as well as sizes of biological [[cell (biology)|cells]] and [[bacteria]],<ref name="Encyclopædia Britannica Online" /> and for grading [[wool]] by the diameter of the fibres.<ref name="NSW Department of Education and Communities">{{cite web|url=http://www.curriculumsupport.education.nsw.gov.au/secondary/technology/7_10/agriculture/course_plans_units/deniliquin/measure_wool_fibre.doc|title=Wool Fibre|work=NSW Department of Education and Communities|format=Word Document download|archive-url=https://web.archive.org/web/20160617004549/http://www.curriculumsupport.education.nsw.gov.au/secondary/technology/7_10/agriculture/course_plans_units/deniliquin/measure_wool_fibre.doc|archive-date=17 June 2016|access-date=18 May 2014}}</ref> The width of a single [[human hair]] ranges from approximately 20 to {{val|200|u=μm}}.

== Examples ==

[[File:How big is 1 micrometer? (10690468113).jpg|thumb|How big is 1 micrometre?]]

Between 1&nbsp;μm and 10&nbsp;μm:
* 1–10&nbsp;μm – length of a typical [[bacterium]]
* 3–8&nbsp;μm – width of strand of [[spider silk|spider web silk]]<ref>{{cite web|url=http://www.earthlife.net/chelicerata/silk.html|title=Spider Silk|author=Ramel|first=Gordon|archive-url=https://web.archive.org/web/20081204080140/http://www.earthlife.net/chelicerata/silk.html|archive-date=4 December 2008 |url-status=live|access-date=14 December 2008|quote=A typical strand of garden spider silk has a diameter of about 0.003 mm ... Dragline silk (about .00032 inch (.008 mm) in ''Nephila'')}}</ref>
* 5&nbsp;μm – length of a typical human [[spermatozoon]]'s head<ref>{{cite journal|last1=Smith|first1=D.J.|last2=Gaffney|first2=E.A.|last3=Blake|first3=J.R.|last4=Kirkman-Brown|first4=J.C.|title=Human sperm accumulation near surfaces: a simulation study|url=http://eprints.maths.ox.ac.uk/860/1/human_sperm.pdf|journal=[[Journal of Fluid Mechanics]]|publisher=[[Cambridge University Press]]|date=25 February 2009|volume=621|pages=295|bibcode=2009JFM...621..289S|doi=10.1017/S0022112008004953|s2cid=3942426|archive-url=https://web.archive.org/web/20131106030353/http://eprints.maths.ox.ac.uk/860/1/human_sperm.pdf|archive-date=6 November 2013|url-status=dead}}</ref>
* 6–8&nbsp;μm – diameter of a typical [[red blood cell]]<ref>{{Cite web |title=Red Blood Cell (RBC) Size Variation |url=https://www.labce.com/spg579126_red_blood_cell_rbc_size_variation.aspx?srsltid=AfmBOopoMxeqbMEfcE32IWxPyhuaqRGeUXTj2QIKupNwUAMGxIpO3UXV |access-date=2025-02-20 |website=www.labce.com}}</ref>
* 10&nbsp;μm – size of [[Hypha|fungal hyphae]]
* about 10&nbsp;μm – size of a [[fog]], [[mist]], or [[cloud]] water droplet

{{For|more examples of things measuring 1 to 10 micrometres|Orders of magnitude (length)#1 micrometre}}

Between 10&nbsp;μm and 100&nbsp;μm:
* about 10–12&nbsp;μm – thickness of [[Plastic wrap|plastic wrap (cling wrap)]]
* 10 to 55&nbsp;μm – width of [[wool]] fibre<ref name="ISTAG">{{cite web|url=http://www.istag.ch/fibres/applications.html|title=Fibreshape applications|publisher=IST - Innovative Sintering Technologies Ltd.|access-date=4 December 2008|quote=Histogram of Fiber Thickness [micrometre]}}</ref>
* 17 to 181&nbsp;μm – diameter of human hair<ref name="Physics Factbook">The [[diameter]] of human hair ranges from 17 to 181 μm. {{cite web
| url=https://hypertextbook.com/facts/1999/BrianLey.shtml
| title=Diameter of a human hair
| first=Brian
| last=Ley
| year=1999
| website=The Physics Factbook
| editor-last=Elert
| editor-first=Glenn
| accessdate=2018-12-08}}</ref>
* 70 to 180&nbsp;μm – thickness of paper
{{For|more examples of things measuring 10 to 100 micrometres|Orders of magnitude (length)#10 micrometres}}

== SI standardization ==
The term ''micron'' and [[Mu (letter)|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.<ref name="BIPM">[http://www.bipm.org/en/committees/cg/CGPM/13-1967/resolution-7/ BIPM - Resolution 7 of the 13th CGPM 1967/68)], "Abrogation of earlier decisions (micron, new candle.)"</ref><ref>{{cite journal | url=https://www.nist.gov/pml/owm/writing-si-metric-system-units | title=Writing with SI (Metric System) Units | journal=NIST | date=13 January 2010 }}</ref> 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.

{{Anchor|Continuing_informal_usage_of_micron}} In [[American English]], the use of ''micron'' may help to differentiate the unit from the [[Micrometer (device)|micrometer]], a measuring device, because the unit's name in [[American and British English spelling differences#-re.2C -er|American spelling]] is a [[homograph]] of the device's name. In spoken English, they are distinguished by pronunciation, as the name of the measuring device is stressed on the second syllable ({{IPAc-en|m|aɪ|ˈ|k|r|ɒ|m|ɪ|t|ər}} {{respell|my|KROM|it|ər}}), whereas the unit name places the stress on the first syllable ({{IPAc-en|ˈ|m|aɪ|k|r|oʊ|m|iː|t|ər}} {{respell|MY|kroh|meet|ər}}).<ref>{{cite LPD|3}}</ref>

The plural of ''micron'' is normally ''microns'', though ''micra'' was occasionally used before 1950.<ref>{{cite book|url=https://books.google.com/books?id=92FOAAAAYAAJ&q=plural+of+micron+microns+or+micra&pg=PA80|title=Proceedings of the Royal Society of Queensland|publisher=H. Pole & Co.|year=1907|series=Part I|volume=XIX|via=Google Books}}</ref><ref>{{cite book|url=https://books.google.com/books?id=lb3RAAAAMAAJ&q=plural+of+micron+microns+or+micra&pg=RA1-PA77|title=The Observer|last1=Bigalow|first1=Edward Fuller|last2=Agassiz Association|year=1905|volume=7–8|via=Google Books}}</ref>

== Symbol ==
{{See also|Micro-#Symbol encoding in character sets}}

The official symbol for the [[metric prefix|SI prefix]] ''micro-'' is a [[mu (letter)|Greek lowercase mu]].<ref>{{Cite web|url=https://www.bipm.org/en/measurement-units/prefixes.html|title=Prefixes of the International System of Units|publisher=[[International Bureau of Weights and Measures]]|access-date=9 May 2016|archive-date=23 May 2018|archive-url=https://web.archive.org/web/20180523190122/https://www.bipm.org/en/measurement-units/prefixes.html|url-status=dead}}</ref> [[Unicode]] has inherited {{unichar|00B5|MICRO SIGN}} from [[ISO/IEC 8859-1]], distinct from the [[code point]] {{unichar|03BC|GREEK SMALL LETTER MU}}. According to the [[Unicode Consortium]], the Greek letter character is preferred,<ref>{{Cite web|url=https://unicode.org/reports/tr25/|title=Unicode Technical Report #25|last1=Beeton|first1=Barbara|last2=Freytag|first2=Asmus|date=30 May 2017|website=Unicode Technical Reports|publisher=[[Unicode Consortium]]|page=11|last3=Sargent|first3=Murray III}}</ref> but implementations must recognize the micro sign as well for [[Unicode compatibility characters|compatibility with legacy character sets]]. Most fonts use the same [[glyph#Typography|glyph]] for the two [[character (computing)|characters]].

Before [[desktop publishing]] became commonplace, it was customary to render the symbol μ in texts produced with mechanical [[typewriter]]s by combining a slightly lowered slash with the letter {{font|u|font=courier}}. For example, "15&nbsp;μm" would appear as "{{font|1=15<span style="letter-spacing:-0.25em"><sub>{{resize|130%|/}}</sub></span>um|font=courier}}". This gave rise in early [[word processing]] to substituting just the letter {{serif|u}} for the symbol if the Greek letter μ was not available, as in "{{serif|15&nbsp;um}}".<ref>{{cite book |title=The American Directory of Writer's Guidelines: A Compilation of Information for Freelancers from More Than 1,300 Magazine Editors and Book Publishers |editor=John C. Mutchler |page=47 |publisher=Quill Driver Books |year=1999 |isbn=978-1-884956-08-9 |url=https://books.google.com/books?id=94MYAAAAIAAJ&q=%22If+you+can-not+access+the+Greek+letter%22+%22use+the+Roman+lower-case+letter+u%22 |edition=2}}</ref>

The Unicode [[CJK Compatibility]] block contains square forms of some Japanese [[katakana]] measure and currency units.
{{Unichar|3348}} corresponds to {{lang|ja|ミクロン}} {{Transliteration|ja|mikuron}}.

== See also ==
* [[Metric prefix]]
* [[Metric system]]
* [[Orders of magnitude (length)]]
* [[Wool measurement]]

== Notes and references ==
{{reflist}}

== External links ==
* {{Wiktionary-inline|micrometre}}

{{SI units of length}}

[[Category:Metre]]
[[Category:Orders of magnitude (length)|-06]]