View source for Millimetre

{{short description|Unit of length 1/1000 of a metre}}

{{further|Metre}}
{{Infobox unit
| bgcolor      = 
| name         = millimetre
| image        = Ruler with millimeter and centimeter marks.png
| caption      = Ruler with millimetre and centimetre marks
| standard     = [[SI]]
| quantity     = [[Length]]
| symbol       = mm
| namedafter   = The [[metric prefix]] ''mille'' ([[Latin]] for "one thousand") and the [[metre]]
| units1   = micrometres
| inunits1     = {{val|1|e=3|ul=μm}} = 1000 [[μm]]
| units2   = centimetres
| inunits2     = {{val|1|e=-1|}} [[centimetre|cm]] = 0.1 [[centimetre|cm]]
| units3   = metres
| inunits3     = {{val|1|e=-3|}} [[metre|m]] = 0.001 [[metre|m]] 
| units4   = kilometres
| inunits4     = {{val|1|e=-6|ul=km}}
| units5   = inches
| inunits5 = {{convert|1|mm|in|disp=out|lk=on|sigfig=5|comma=gaps}}
| units6   = feet
| inunits6 = {{convert|1|mm|ft|disp=out|lk=on|sigfig=5|comma=gaps}}
}}
[[file:EM Spectrum Properties edit.svg|thumb|330px|Different lengths as in respect to the [[electromagnetic spectrum]], measured by the metre and its derived scales. The [[microwave]] is between 1 meter to 1 '''millimeter'''.]]
The '''millimetre''' ([[American and British English spelling differences#-re, -er|international spelling]]; [[International System of Units|SI unit]] symbol '''mm''') or '''millimeter''' ([[American and British English spelling differences#-re, -er|American spelling]]) is a [[Units of measurement|unit]] of [[length]] in the [[International System of Units]] (SI), [[Equality (mathematics)|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.

One millimetre is equal to {{val|1000}} [[micrometre]]s or {{val|1,000,000}} [[nanometre]]s.
Since an [[inch]] is officially defined as exactly 25.4 millimetres, a millimetre is equal to exactly {{frac|5|127}} (≈ 0.03937) of an inch.

==Definition==
Since 1983, the [[metre]] has been defined as "the length of the path travelled by [[speed of light|light]] in [[vacuum]] during a time interval of {{sfrac|{{val|299,792,458}}}} of a [[second]]".<ref>{{cite web |url=http://www.bipm.org/en/CGPM/db/17/1/ |title=17th General Conference on Weights and Measures (1983), Resolution 1. |access-date=3 December 2013 |publisher=International Bureau of Weights and Measures}}</ref> A millimetre, {{sfrac|1000}} of a metre, is therefore the distance travelled by light in {{sfrac|{{val|299,792,458,000}}}} 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 [[CJK Compatibility|compatibility]] with [[Chinese character|Chinese]], [[Japanese language|Japanese]] and [[Korean language|Korean]] ([[CJK characters|CJK]]) characters, [[Unicode subscripts and superscripts|Unicode]] has symbols for:<ref name=comp>{{cite web |url=https://www.unicode.org/charts/PDF/U3300.pdf |title=CJK Compatibility |publisher=unicode.org |access-date=3 December 2013}}</ref>
* millimetre - {{unichar|339C|SQUARE MM}}
* square millimetre - {{unichar|339F|SQUARE MM SQUARED}}
* cubic millimetre {{unichar|33A3|SQUARE MM CUBED}}

In Japanese typography, these square symbols are used for laying out unit symbols without distorting the grid layout of text characters.

==Measurement==
On a metric ruler, the smallest measurements are normally millimetres.<ref>{{cite web |url=http://www.onlineconversion.com/faq_05.htm |title=How do I read a ruler? |publisher=onlineconversion.com |access-date=3 December 2013}}</ref> High-quality engineering rulers may be graduated in increments of 0.5&nbsp;mm. Digital [[Calipers#Digital_caliper|callipers]] are commonly capable of reading increments as small as 0.01&nbsp;mm.<ref>{{cite web |url=http://www.tresnainstrument.com/accuracy_of_calipers.html |title=Accuracy of Calipers |publisher=TresnaInstrument.com |access-date=3 December 2013}}</ref>

[[Microwave]]s with a frequency of 300&nbsp;GHz have a wavelength of 1&nbsp;mm. Using frequencies between 30&nbsp;GHz and 300&nbsp;GHz for data transmission, in contrast to the 300&nbsp;MHz to 3&nbsp;GHz normally used in mobile devices, has the potential to allow data transfer rates of 10&nbsp;[[gigabit]]s per second.<ref>{{cite book |url=https://books.google.com/books?id=kCmyPdxme1AC&pg=PP15 |title=Millimeter Wave Communication Systems |isbn=9781118102756 |last1=Huang |first1=Kao-Cheng |last2=Wang |first2=Zhaocheng |year=2011|publisher=John Wiley & Sons }}</ref>

The smallest dimension the human eye can resolve is around 0.02 to 0.04&nbsp;mm, approximately the width of a thin [[human hair]].<ref>{{cite web |url=http://sciencefocus.com/qa/how-small-can-naked-eye-see |title=How Small Can the Naked Eye See? |publisher=Focus Magazine |access-date=3 December 2013}}</ref> A sheet of [[paper]] is typically between 0.07&nbsp;mm and 0.18&nbsp;mm thick, with ordinary printer paper or copy paper approximately 0.1&nbsp;mm thick.<ref>{{cite web
| url=https://hypertextbook.com/facts/2001/JuliaSherlis.shtml
| title=Thickness of a piece of paper
| first=Juliya
| last=Sherlis
| year=2001
| website=The Physics Factbook
| editor-last=Elert
| editor-first=Glenn
| accessdate=2022-01-21
}}</ref>

==See also==
{{Wiktionary|millimetre}}
* [[Metric system]]
* [[Orders of magnitude (length)]]
* [[Submillimetre astronomy]]

==References==
{{reflist}}

{{SI units of length}}

[[Category:Metre]]
[[Category:Orders of magnitude (length)|-03]]
[[Category:1000 (number)]]