View source for Inch

{{Infobox unit
| image    = Inch tape.jpg
| caption  = A measuring tape with inches
| symbol   = in
| symbol2 = {{pprime}} (the double prime)<ref name="Unicode-U2000">{{cite web |url=http://unicode.org/charts/PDF/U2000.pdf |title=The Unicode Standard 12.1 — General Punctuation ❰ Range: 2000—206F ❱ |author=Unicode Consortium |date=2019 |website=Unicode.org}}</ref>
| standard = [[Imperial units|Imperial]]/US units
| quantity = [[Physics:Length|Length]]
| units2   = [[Metric system|Metric]] (SI) units
| inunits2 = 25.4&nbsp;[[Millimetre|mm]]
| units1   = Imperial/US units
| inunits1 = {{sfrac|36}}&nbsp;yd or {{sfrac|12}}&nbsp;[[Foot (unit)|ft]]
}}
[[File:3 inch zoll pipe.jpg|thumb|A fire hydrant marked as 3-inch]]

The '''inch''' (symbol: '''in''' or '''[[Prime (symbol)|{{pprime}}]]''') is a unit of [[Physics:Length|length]] in the [[Imperial units|British Imperial]] and the United States customary [[System of measurement|systems of measurement]]. It is equal to {{sfrac|1|36}} [[Yard|yard]] or {{sfrac|1|12}} of a [[Foot (unit)|foot]]. Derived from the [[Uncia (unit)|Roman uncia]] ("twelfth"), the word ''inch'' is also sometimes used to translate similar units in other measurement systems, [[Anthropic units|usually understood as deriving]] from the width of the human thumb.

Standards for the exact length of an inch have varied in the past, but since the adoption of the international yard during the 1950s and 1960s the inch has been based on the [[Metric system|metric system]] and defined as exactly 25.4{{nbsp}}[[Millimetre|mm]].

{{anchor|Etymology}}
==Name==
The English word "inch" ({{langx|ang|ynce}}) was an early borrowing from [[Social:Latin|Latin]] ''{{lang|la|uncia}}'' ("one-twelfth; Roman inch; Roman ounce").<ref>{{citation |contribution=inch, ''n.<sup>1</sup>'' |title=Oxford English Dictionary |location=Oxford |publisher=Oxford University Press }}.</ref> The vowel change from Latin {{IPA|/u/}} to Old English {{IPA|/y/}} (which became Modern English {{IPA|/ɪ/}}) is known as umlaut. The consonant change from the Latin {{IPA|/k/}} (spelled ''c'') to English {{IPA|/tʃ/}} is palatalisation. Both were features of Old English phonology; see {{section link|Phonological history of Old English|Palatalization}} and {{section link|Germanic umlaut|I-mutation in Old English}} for more information.

"Inch" is cognate with "[[Ounce|ounce]]" ({{langx|ang|ynse}}), whose separate pronunciation and spelling reflect its reborrowing in [[Social:Middle English|Middle English]] from [[Social:Anglo-Norman language|Anglo-Norman]] ''unce'' and ''ounce''.<ref>{{citation |contribution=ounce, ''n.<sup>1</sup>'' |title=Oxford English Dictionary |location=Oxford |publisher=Oxford University Press }}.</ref>

In many other European languages, the word for "inch" is the same as or derived from the word for "thumb", as a man's thumb is about an inch wide (and this was even sometimes used to define the inch<ref>{{Cite web|url=https://www.britannica.com/science/inch|title=Inch {{!}} unit of measurement|website=Encyclopedia Britannica|language=en|access-date=28 March 2019}}</ref>). In the [[Social:Dutch language|Dutch language]] a term for inch is ''engelse duim'' (english thumb).<ref>{{cite web |url=https://onzetaal.nl/taalloket/duim-lengtemaat |title=duim - lengtemaat |publisher=Genootschap Onze Taal|access-date=22 October 2022}}</ref><ref>{{cite web |url=https://www.omrekenen.nl/duim/ |title=duim |date=24 May 2020 |access-date=22 October 2022}}</ref> Examples include {{langx|ca|polzada}} ("inch") and ''{{lang|ca|polze}}'' ("thumb"); {{langx|cs|palec}} ("thumb"); Danish and {{langx|no|tomme}} ("inch") ''{{lang|no|tommel}}'' ("thumb"); {{langx|nl|duim}} (whence {{langx|af|duim}} and {{langx|ru|дюйм}}); {{langx|fr|pouce}}; {{lang-ka|დუიმი}}, {{langx|hu|hüvelyk}}; {{langx|it|pollice}}; {{langx|pt|polegada}} ("inch") and ''{{lang|pt|polegar}}'' ("thumb"); ("duim"); {{langx|sk|palec}} ("thumb"); {{langx|es|pulgada}} ("inch") and ''{{lang|es|pulgar}}'' ("thumb"); and {{langx|sv|tum}} ("inch") and ''tumme'' ("thumb").

== Usage ==
===Imperial or hybrid countries===
The inch is a commonly used customary unit of length in the United States,<ref>{{cite web |url=http://corpus.byu.edu/coca/ |title=Corpus of Contemporary American English |website=Brigham Young University |location=US |access-date=5 December 2011}} lists 24,302 instances of inch(es) compared to 1548 instances of centimeter(s) and 1343 instances of millimeter(s).</ref> Canada,<ref>{{cite web |url=http://laws-lois.justice.gc.ca/PDF/W-6.pdf |title=Weights and Measures Act |page=37 |via=Justice Laws Website |location=Canada |year=1985 |access-date=11 January 2018}}</ref><ref>{{cite web |url=http://laws-lois.justice.gc.ca/eng/acts/W-6/page-2.html#h-4/ |title=Weights and Measures Act |page=2 |via=Justice Laws Website |location=Canada |date=1 August 2014 |access-date=18 December 2014}} Canadian units (5) The Canadian units of measurement are as set out and defined in Schedule II, and the symbols and abbreviations therefore are as added pursuant to subparagraph 6(1)(b)(ii).</ref> and the United Kingdom.<ref name=bis/> For the United Kingdom, guidance on public sector use states that, since 1 October 1995, without time limit, the inch (along with the foot) is to be used as a primary unit for road signs and related measurements of distance (with the possible exception of clearance heights and widths)<ref>{{cite web |url=http://www.legislation.gov.uk/uksi/2002/3113/schedule/2/made |title=The Traffic Signs Regulations and General Directions 2002 - No. 3113 - Schedule 2 - Regulatory Signs |publisher=The National Archives |location=UK |year=2002 |access-date=25 April 2013}}</ref> and may continue to be used as a secondary or supplementary indication following a metric measurement for other purposes.<ref name=bis>{{cite web |url=http://www.bis.gov.uk/assets/bispartners/nmo/docs/legislation/legislation/units-of-measurement/gnotes-for-public-sector-on-use-of-metric.pdf |archive-url=https://web.archive.org/web/20110704232045/http://www.bis.gov.uk/assets/bispartners/nmo/docs/legislation/legislation/units-of-measurement/gnotes-for-public-sector-on-use-of-metric.pdf |url-status=dead |archive-date=4 July 2011 |title=Guidance Note on the use of Metric Units of Measurement by the Public Sector |publisher=Department for Business Innovation and Skills |location=UK |year=2007 |access-date=12 December 2014}}</ref>

===Worldwide===
Inches are used for display screens (e.g. [[Engineering:Television|television]]s and [[Engineering:Computer monitor|computer monitor]]s) worldwide. It is the official [[Japanese units of measurement#Imperial units|Japanese standard]] for electronic parts, especially display screens, and is the industry standard throughout continental Europe for display screens. However, many countries across the world which use the metric system, such as France<ref>{{Cite web|url=https://www.manutan.fr/fr/maf/televiseur#productBeginIndex:0&orderBy:5&|title=Téléviseur|publisher=Manutan|language=fr|access-date=25 September 2025}}</ref>, Germany<ref>{{Cite web|url=https://www.otto.de/technik/fernseher/|title=Fernseher|publisher=Otto GmbH|language=de|access-date=28 December 2023}}</ref> or [[Place:Turkey|Turkey]]<ref>{{Cite web|url=https://www.vatanbilgisayar.com/televizyon/|title=Televizyon|publisher=Vatan Bilgisayar|language=tr|access-date=25 September 2025}}</ref> often supplement it with centimetres in advertisements, catalogues and packages.

Inches are commonly used to specify the diameter of vehicle wheel rims, and the corresponding inner diameter of tyres in tyre codes.<ref>{{Cite web|url=https://www.michelin.co.uk/auto/advice/tyre-basics/tyre-markings-explained|title=Tyre Sizing|access-date=14 May 2025}}</ref>

===Technical details===

The international standard symbol for inch is '''in''' (see [[Physics:ISO 31-1|ISO 31-1]], Annex A) but traditionally the inch is denoted by a [[Prime (symbol)|double prime]], which is often approximated by a double quote symbol, and the [[Foot (unit)|foot]] by a [[Prime (symbol)|prime]], which is often approximated by an apostrophe. For example; ''{{nowrap|three feet, two inches}}'' can be written as 3{{prime}} 2{{pprime}}. (This is akin to how the [[Minute|first]] and [[Second|second]] "cuts" of the [[Hour|hour]] are likewise indicated by prime and double prime symbols, and also the [[Minute of arc|first]] and second cuts of the [[Degree (angle)|degree]].)

Subdivisions of an inch are typically written using [[Dyadic rational|dyadic fractions]] with odd number numerators; for example, ''{{nowrap|two and three-eighths of an inch}}'' would be written as {{sfrac|2|3|8}}{{pprime}} and not as 2.375{{pprime}} nor as {{sfrac|2|6|16}}{{pprime}}.  However, for engineering purposes fractions are commonly given to three or four places of decimals and have been for many years.<ref>{{cite book|last1=Flatchet|first1=E|last2=Petiet|first2=J|title=The student's guide to the locomotive engine|url=https://archive.org/details/studentsguideto00ptgoog|date=1849|publisher=John Williams and Co|page=xi|quote=One Metre is equal to ... 30.371 inches"}}</ref><ref>{{cite book|last=Parkinson|first=A C |title=Intermediate Engineering Drawing|date=1967|edition=sixth|page=11|quote=The basic major dia is actually 1.309 in.}}</ref>

=== Equivalents===
{{calculator|id=in|type=number|size=4|default=1}} international inch is equal to:
{{div col|colwidth=30em}}
* {{calculator|id=cm|type=plain|default=2.54|formula=in*2.54|decimals=1|NaN-text=0}} centimeters (1 inch is exactly 2.54&nbsp;cm)
* {{calculator|id=mm|type=plain|default=25.4|formula=in*25.4|decimals=1|NaN-text=0}} [[Millimetre|millimetre]]s (1 inch is exactly 25.4&nbsp;mm)
* {{sfrac|{{calculator|id=numeratorfeet|type=plain|default=1|formula=in|NaN-text=0}}|12}} or {{calculator|id=feet|type=plain|default=0.08333|formula=in/12|NaN-text=0}} [[Foot (unit)|feet]] (approximately)
* {{sfrac|{{calculator|id=numeratoryards|type=plain|default=1|formula=in|NaN-text=0}}|36}} or {{calculator|id=yard|type=plain|default=0.02777|formula=in/36|NaN-text=0}} [[Yard|yard]]s (approximately)
* {{calculator|id=tenths|type=plain|default=10000|formula=in*10000|NaN-text=0}} '[[Thousandth of an inch#Tenths|tenths]]'{{efn|A tenth of a [[Thousandth of an inch|thou]], used in [[Engineering:Machining|machining]].}}
* {{calculator|id=thou|type=plain|default=1000|formula=in*1000|NaN-text=0}} [[Thousandth of an inch|thou]]{{efn|Used in [[Engineering:Machining|machining]] and papermaking.}} or mil{{efn|Formerly used in [[American English]] but now often avoided to prevent confusion with [[Millimetre|millimetre]]s.}}
* {{calculator|id=points|type=plain|default=100|formula=in*100|NaN-text=0}} points{{efn|Used by the Australian Bureau of Meteorology for measuring rainfall until 1973<ref>{{cite web |url=http://www.bom.gov.au/climate/cdo/about/definitionsrain.shtml |title=Climate Data Online – definition of rainfall statistics |publisher=Bureau of Meteorology |location=Australia |access-date=10 June 2012}}</ref>}} or gries{{efn|name=locke|Part of [[Biography:John Locke|John Locke]]'s proposal for decimalization of English measures<ref>{{citation |last=Locke |first=John |display-authors=0 |contribution=Of Human Understanding |page=[https://books.google.com/books?id=3n8PAAAAQAAJ&pg=PA293 293] |title=The Works of John Locke Esq., ''Vol. I'' |location=London |publisher=John Churchill |date=1714 |url=https://books.google.com/books?id=3n8PAAAAQAAJ }}.</ref>}}
* {{calculator|id=pointsTypography|type=plain|default=72|formula=in*72|NaN-text=0}} [[Point (typography)|PostScript point]]s{{efn|The typographic point was originally {{sfrac|1|9}} of the height of a (capital) letter (cap height) but later acquired a number of different absolute definitions; see [[Point (typography)#History|Point (typography) § History]] for details.}}
* {{calculator|id=line10|type=plain|default=10|formula=in*10|NaN-text=0}},{{efn|Used in gunmaking.}}<ref group=lower-alpha name=locke/> {{calculator|id=line12|type=plain|default=12|formula=in*12|NaN-text=0}},{{efn|Used in [[Biology:Botany|botany]].}} or {{calculator|id=line40|type=plain|default=40|formula=in*40|NaN-text=0}}{{efn|Used in button manufacturing.}} [[Line (unit)|lines]]
* {{calculator|id=pica|type=plain|default=6|formula=in*6|NaN-text=0}} [[Pica (typography)|computer picas]]{{efn|Used in typography.}}
* {{calculator|id=barleycorn|type=plain|default=3|formula=in*3|NaN-text=0}} [[Barleycorn (unit)|barleycorns]]{{efn|Used in American and British [[Medicine:Shoe size|shoe size]]s.}}
* {{calculator|id=ussurveyin|type=plain|default=0.999998|formula=in*0.999998|NaN-text=0}} US survey inches
* {{sfrac|{{calculator|id=numeratorPalm|type=plain|default=1|formula=in|NaN-text=0}}|3}} or {{calculator|id=palms|type=plain|default=0.333|formula=in/3|NaN-text=0}} [[Palm (unit)|palms]] (approximately)
* {{sfrac|{{calculator|id=numeratorhand|type=plain|default=1|formula=in|NaN-text=0}}|4}} or {{calculator|id=hand|type=plain|default=0.25|formula=in/4|NaN-text=0}} [[Hand (unit)|hands]]{{efn|Used in measuring the height of [[Biology:Horse|horse]]s.}}
{{Div col end}}

== History ==
[[File:Inch converter.jpg|thumb|Mid-19th-century tool for converting between different standards of the inch]]

The earliest known reference to the inch in England is from the ''Laws of Æthelberht'' dating to the early 7th century, surviving in a single manuscript, the ''Textus Roffensis'' from 1120.<ref name="GoetzJarnut2003">{{cite book |last1=Goetz |first1=Hans-Werner |last2=Jarnut |first2=Jörg |last3=Pohl |first3=Walter |title=Regna and Gentes: The Relationship Between Late Antique and Early Medieval Peoples and Kingdoms in the Transformation of the Roman World |url=https://books.google.com/books?id=RovRlJkrncEC&pg=PA33 |year=2003 |publisher=BRILL |isbn=978-90-04-12524-7 |page=33}}</ref> Paragraph LXVII sets out the fine for wounds of various depths: one inch, one shilling; two inches, two shillings, etc.{{efn|{{langx|ang|Gif man þeoh þurhstingð, stice ghwilve vi scillingas. Gife ofer ynce, scilling. æt twam yncum, twegen. ofer þry, iii scill.}}  Translation (taken from {{harvnb|Attenborough|1922|p=13}}): If a thigh is pierced right through, 6 shillings compensation shall be paid for each stab.  For a stab over an inch [deep], 1 shilling; for a stab between 2 and 3 inches, 2 shillings; for a stab over 3 inches 3 shillings.<ref name="Wilkins1871">{{cite book |last=Wilkins |first=David |title=Councils and Ecclesiastical Documents Relating to Great Britain and Ireland: English church during the Anglo-Saxon period: A.D. 595-1066. |url=https://archive.org/details/councilsandeccl04wilkgoog |access-date=18 December 2014 |year=1871 |publisher=Clarendon Press |location=Oxford, UK |page=[https://archive.org/details/councilsandeccl04wilkgoog/page/n72 48]}}</ref><ref name="Duncan1984">{{cite book |last=Duncan |first=Otis Dudley |author-link=Otis Dudley Duncan |title=Notes on social measurement: historical and critical |url=https://books.google.com/books?id=5c459mDugI0C&pg=PA87 |year=1984 |publisher=Russell Sage Foundation |location=US |isbn=978-0-87154-219-9 |page=87}}</ref>}}

An Anglo-Saxon unit of length was the [[Barleycorn (unit)|barleycorn]]. After 1066, 1&nbsp;inch was equal to 3 barleycorns, which continued to be its legal definition for several centuries, with the barleycorn being the base unit.<ref name=Klein>{{cite book |title=The world of measurements: masterpieces, mysteries and muddles of metrology |url=https://archive.org/details/worldofmeasureme0000klei |url-access=registration |first=H. Arthur |last=Klein |publisher=Simon and Schuster |location=New York, US |year=1974|isbn=9780671215651 }}</ref> One of the earliest such definitions is that of 1324, where the legal definition of the inch was set out in a statute of Edward II of England, defining it as "three grains of [[Biology:Barley|barley]], dry and round, placed end to end, lengthwise".<ref name=Klein />

Similar definitions are recorded in both English and Welsh medieval law tracts.<ref>{{cite book |pages=310 |title=Northumbria's Golden Age |first1=Jane |last1=Hawkes |first2=Susan |last2=Mills |publisher=Sutton |location=UK |year=1999 |isbn=978-0-7509-1685-1}}</ref> One, dating from the first half of the 10th century, is contained in the Laws of Hywel Dda which superseded those of Dyfnwal, an even earlier definition of the inch in Wales. Both definitions, as recorded in ''Ancient Laws and Institutes of Wales'' (vol i., pp.&nbsp;184, 187, 189), are that "three lengths of a barleycorn is the inch".<ref>{{cite book |title=The Traditionary Annals of the Cymry |url=https://archive.org/details/traditionaryann00willgoog |first=John |last=Williams |chapter=The civil arts – mensuration |pages=[https://archive.org/details/traditionaryann00willgoog/page/n253 243]&ndash;245 |location=Tenby, UK |publisher=R. Mason |year=1867}}</ref>

King David I of Scotland in his Assize of Weights and Measures (c. 1150) is said to have defined the Scottish inch as the width of an average man's thumb at the base of the nail, even including the requirement to calculate the average of a small, a medium, and a large man's measures.<ref name="Swinton1789">{{cite book |last1=Swinton |first1 = John| title=A proposal for uniformity of weights and measures in Scotland |url=https://books.google.com/books?id=kHhbAAAAQAAJ&pg=PA134 |year=1789 |publisher=printed for Peter Hill |page=134}}</ref> However, the oldest surviving manuscripts date from the early 14th century and appear to have been altered with the inclusion of newer material.<ref name="GemmillMayhew2006">{{cite book |first1=Elizabeth |last1=Gemmill |first2=Nicholas |last2=Mayhew |title=Changing Values in Medieval Scotland: A Study of Prices, Money, and Weights and Measures |url=https://books.google.com/books?id=t1hmrzOSEagC&pg=PA113 |date=22 June 2006 |publisher=Cambridge University Press| location=UK |isbn=978-0-521-02709-0 |page=113}}</ref>

In 1814, Charles Butler, a mathematics teacher at Cheam School, recorded the old legal definition of the inch to be "three grains of sound ripe barley being taken out the middle of the ear, well dried, and laid end to end in a row", and placed the barleycorn, not the inch, as the base unit of the English Long Measure system, from which all other units were derived.<ref name=Butler>{{cite book |pages=[https://archive.org/details/aneasyintroduct01butlgoog/page/n64 61] |title=An Easy Introduction to the Mathematics |url=https://archive.org/details/aneasyintroduct01butlgoog |first=Charles |last=Butler |location=Oxford, UK |publisher=Bartlett and Newman |year=1814}}</ref> John Bouvier similarly recorded in his 1843 law dictionary that the barleycorn was the fundamental measure.<ref>{{cite encyclopedia |first=John |last=Bouvier |article=Barleycorn |year=1843 |encyclopedia=A Law Dictionary: With References to the Civil and Other Systems of Foreign Law |pages=188 |location=Philadelphia, US |publisher=T. & J. W. Johnson}}</ref> Butler observed, however, that "[a]s the length of the barley-corn cannot be fixed, so the inch according to this method will be uncertain", noting that a standard inch measure was now [i.e. by 1843] kept in the Exchequer chamber, Guildhall, and ''that'' was the legal definition of the inch.<ref name=Butler />

This was a point also made by George Long in his 1842 ''Penny Cyclopædia'', observing that standard measures had since surpassed the barleycorn definition of the inch, and that to recover the inch measure from its original definition, in case the standard measure were destroyed, would involve the measurement of large numbers of barleycorns and taking their average lengths. He noted that this process would not perfectly recover the standard, since it might introduce errors of anywhere between one hundredth and one tenth of an inch in the definition of a yard.<ref>{{cite encyclopedia |page=436 |article=Weights & Measures, Standard |encyclopedia=The Penny Cyclopædia of the Society for the Diffusion of Useful Knowledge |first=George |last=Long |year=1842 |location=London, UK |publisher=Charles Knight & Co.}}</ref>

Before the adoption of the [[International yard and pound|international yard and pound]], various definitions were in use. In the United Kingdom and most countries of the [[Organization:Commonwealth of Nations|British Commonwealth]], the inch was defined in terms of the [[Yard#19th-century Britain|Imperial Standard Yard]]. The United States adopted the conversion factor 1 metre = 39.37 inches by an act in 1866.<ref>{{cite book
  | first1      = Lewis V
  | last1       = Judson
  | title       = Weights and Measures Standards of the United States - a brief history - NBS publication 447
  | url         = https://books.google.com/books?id=GXWfglKg11MC&q=1838+gallon+231+congress&pg=PA8
  | page        = 10&ndash;11
  | publisher   = United States Department of Commerce
  | date        = October 1963
}}</ref> In 1893, Mendenhall ordered the physical realization of the inch to be based on the international prototype metres numbers 21 and 27, which had been received from the CGPM, together with the previously adopted conversion factor.<ref name="Mendenhall">{{cite web |author=T. C. Mendenhall, Superintendent of Standard Weights and Measures |url=http://physics.nist.gov/Pubs/SP447/app3.pdf |date=5 April 1893 |title=Appendix&nbsp;6 to the Report for 1893 of the Coast and Geodetic Survey |url-status=dead |archive-url=https://web.archive.org/web/20120930180925/http://physics.nist.gov/Pubs/SP447/app3.pdf |archive-date=30 September 2012 }}</ref>

As a result of the definitions above, the U.S. inch was effectively defined as 25.4000508&nbsp;mm (with a reference temperature of 68 degrees Fahrenheit) and the UK inch at 25.399977&nbsp;mm (with a reference temperature of 62 degrees Fahrenheit). When Carl Edvard Johansson started manufacturing gauge blocks in inch sizes in 1912, Johansson's compromise was to manufacture gauge blocks with a nominal size of 25.4mm, with a reference temperature of 20 degrees Celsius, accurate to within a few parts per million of both official definitions. Because Johansson's blocks were so popular, his blocks became the ''de facto'' standard for manufacturers internationally,<ref name=":0">{{Cite web|url=http://mitutoyo.com/wp-content/uploads/2013/07/E12016-History-of-The-Gage-Block.pdf#page=8|title=The History of Gauge Blocks|date=2013|website=mitutoyo.com|publisher=Mitutoyo Corporation|page=8|access-date=1 February 2020}}</ref><ref>{{Cite book|url=https://books.google.com/books?id=3rUaAQAAMAAJ&pg=PA293|title=Industrial Standardization and Commercial Standards Monthly|date=October 1943|language=en|access-date=1 February 2020|first=John|last=Gaillard|page=293}}</ref> with other manufacturers of gauge blocks following Johansson's definition by producing blocks designed to be equivalent to his.<ref>{{Cite book|url=https://books.google.com/books?id=NiEEAQAAIAAJ&pg=PA200|title=Measures for Progress. NIST Special Publication, isue 275.|last=Cochrane|first=Rexmond C.|publisher=U.S. Government Printing Office|year=1966|pages=200|language=en|lccn=65-62472}}</ref>

In 1930, the British Standards Institution adopted an inch of exactly 25.4&nbsp;mm. The American Standards Association followed suit in 1933. By 1935, industry in 16 countries had adopted the "industrial inch" as it came to be known,<ref name="MeasuresStandards1936">{{cite conference |url={{google books|id=WDgJAQAAMAAJ|page=RA3-PA4|plain-url=yes}} |conference=National Twenty-Eight Conference on Weights and Measures |title=The Viewpoint of industry concerned with interchangeable manufacturing toward the proposal to standardize the inch |first=Herbert B. |last=Lewis  |publisher=National Bureau of Standards |location=US |year=1936 |page=4 |access-date=2 August 2012}}</ref><ref name="WandmacherJohnson1995">{{cite book |last1=Wandmacher |first1=Cornelius |last2=Johnson |first2=Arnold Ivan |title=Metric Units in Engineering--going SI: How to Use the International Systems of Measurement Units (SI) to Solve Standard Engineering Problems |url=https://books.google.com/books?id=v5AGFgpcK_wC&pg=PA265 |year=1995 |publisher=ASCE Publications|isbn=978-0-7844-0070-8|page=265}}</ref> effectively endorsing Johansson's pragmatic choice of conversion ratio.<ref name=":0" />

In 1946, the Commonwealth Science Congress recommended a yard of exactly 0.9144&nbsp;metres for adoption throughout the British Commonwealth. This was adopted by Canada in 1951;<ref>{{cite journal|title=Announcement on the International Yard and Pound|first=L. E.|last=Howlett|date=1 January 1959|journal=Canadian Journal of Physics|volume=37|issue=1|pages=84|doi=10.1139/p59-014|bibcode=1959CaJPh..37...84H}}</ref><ref name="MeasuresStandards1957">{{cite book |author1=National Conference on Weights and Measures |author2=United States. Bureau of Standards |author3=National Institute of Standards and Technology (US) |title=Report of the ... National Conference on Weights and Measures |url=https://books.google.com/books?id=CEgJAQAAMAAJ&pg=PA45 |year=1957 |publisher=US Department of Commerce, Bureau of Standards |pages=45–6}}</ref> the United States on 1 July 1959;<ref name="FR59-5442">{{cite web |last1=Astin |first1=A.V. |last2=Karo |first2=H. A. |last3=Mueller |first3=F.H. |date=25 June 1959 |url=http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf |title=Refinement of Values for the Yard and the Pound |publisher=US Federal Register}}</ref><ref name="Standards1959">{{cite book |author=United States. National Bureau of Standards |author-link=National Institute of Standards and Technology |title=Research Highlights of the National Bureau of Standards |url=https://books.google.com/books?id=4aWN-VRV1AoC&pg=PA13 |year=1959 |publisher=US Department of Commerce, National Bureau of Standards |page=13}}</ref><ref name="JudsonStandards1976">{{cite book|author1=Lewis Van Hagen Judson|author2=United States. National Bureau of Standards|author-link2=National Institute of Standards and Technology|title=Weights and measures standards of the United States: a brief history|url=https://archive.org/details/bub_gb_o4XQ7KCv5E0C|access-date=16 September 2012|year=1976|publisher=Dept. of Commerce, National Bureau of Standards : for sale by the Supt. of Docs., U.S. Govt. Print. Off.|pages=[https://archive.org/details/bub_gb_o4XQ7KCv5E0C/page/n36 30]–1}}</ref> Australia in 1961,<ref>Statutory Rule No. 142.</ref> effective 1 January 1964;<ref>[http://www.comlaw.gov.au/Details/C2004L00578 Australian Government ComLaw ''Weights and Measures (National Standards) Regulations - C2004L00578'']</ref> and the United Kingdom in 1963,<ref>Weights and Measures Act of 1963.</ref> effective on 1 January 1964.<ref>{{cite web |url=http://www.bailii.org/ew/cases/EWHC/Admin/2002/195.html |title=Thoburn v Sunderland City Council [2002] EWHC 195 (Admin) |date=18 February 2002 |publisher=England and Wales High Court |via=British and Irish Legal Information Institute}}</ref> The new standards gave an inch of exactly 25.4&nbsp;mm, 1.7 millionths of an inch longer than the old imperial inch and 2 millionths of an inch shorter than the old US inch.<ref>{{cite web |url=http://www.npl.co.uk/reference/faqs/on-what-basis-is-one-inch-exactly-equal-to-25.4-mm-has-the-imperial-inch-been-adjusted-to-give-this-exact-fit-and-if-so-when-(faq-length) |archive-url= https://web.archive.org/web/20130126164151/http://www.npl.co.uk/reference/faqs/on-what-basis-is-one-inch-exactly-equal-to-25.4-mm-has-the-imperial-inch-been-adjusted-to-give-this-exact-fit-and-if-so-when-(faq-length) | archive-date = 26 January 2013 |title=On what basis is one inch exactly equal to 25.4 mm? Has the imperial inch been adjusted to give this exact fit and if so when? |publisher=National Physical Laboratory |date=25 March 2010 |access-date=5 April 2013}}</ref><ref name=Astin/>

==Related units==

===US survey inches===
The United States retained the {{sfrac|1|39.37}}-metre definition for surveying, producing a 2 millionth part difference between standard and US survey inches.<ref name = Astin>A. V. Astin & H. Arnold Karo, (1959), [http://www.ngs.noaa.gov/PUBS_LIB/FedRegister/FRdoc59-5442.pdf ''Refinement of values for the yard and the pound''], Washington DC: National Bureau of Standards, republished on National Geodetic Survey web site and the Federal Register (Doc. 59-5442, Filed, 30 June 1959, 8:45&nbsp;am)</ref> This is approximately {{sfrac|1|8}}&nbsp;inch per mile; 12.7 kilometres is exactly {{Nts|500000}} standard inches and exactly {{Nts|499999}} survey inches. This difference is substantial when doing calculations in [[Earth:State Plane Coordinate System|State Plane Coordinate System]]s with coordinate values in the hundreds of thousands or millions of feet.

In 2020, the [[National Institute of Standards and Technology]] announced that the U.S. survey foot would "be phased out" on 1 January 2023 and be superseded by the international foot (also known as the foot) equal to 0.3048&nbsp;metres exactly, for all further applications.<ref>{{Cite web|url=https://www.nist.gov/pml/us-surveyfoot|title=U.S. Survey Foot|last=Materese|first=Robin|date=26 July 2019|website=NIST|language=en|access-date=1 February 2020}}</ref> This implies that the survey inch was replaced by the international inch.

===Continental inches===
Before the adoption of the metric system, several European countries had customary units whose name translates into "inch". The French ''pouce'' measured roughly 27.0&nbsp;mm, at least when applied to describe the calibre of artillery [[Engineering:Cannon|pieces]]. The [[Physics:Dutch units of measurement|Amsterdam foot]] (''voet'') consisted of 11 Amsterdam inches (''duim''). The Amsterdam foot is about 8% shorter than an English foot.<ref>*{{cite book
|url = {{google book|id=XYVbAAAAQAAJ|page=166|plain-url=yes}}
|title = Allereerste Gronden der Cijferkunst
|first = Jacob
|last = de Gelder
|location = The Hague <!-- Dutch: 's Gravenhage and Amsterdam -->
|language = nl
|year = 1824
|pages = 166
|publisher = de Gebroeders van Cleef
|trans-title=Introduction to Numeracy
|access-date = 10 April 2022}}
</ref>

=== Scottish inch ===
The now obsolete Scottish inch ({{langx|gd|òirleach}}), {{sfrac|12}} of a Scottish foot, was about 1.0016 imperial inches (about {{convert|1.0016|in|mm|disp=output only}}).<ref>{{cite web
|url =https://dsl.ac.uk/entry/snd/snds3729
|title = Dictionary of the Scots Language
|publisher = Scottish Language Dictionaries
|location = Edinburgh
|access-date =22 January 2020}}</ref>

== See also ==
{{div col|colwidth=23em}}
* [[English units of measurement|English units]]
* [[Square inch]] and [[Cubic inch]]
* [[International yard and pound]]
* [[Anthropic units]]
* [[Pyramid inch]]
* [[Digit (unit)|Digit]] and [[Line (unit)|Line]]
{{Div col end}}

==Notes==
{{Notelist|2}}

== References ==
===Citations===
{{Reflist|30em}}

===Bibliography===
* {{Citation | last = Attenborough | first = F. L. |  title =  The Laws of the Earliest English Kings | edition = Llanerch Press Facsimile Reprint 2000 | location = Cambridge | publisher = Cambridge University Press | year = 1922 |  isbn = 978-1-86143-101-1 |url=https://archive.org/details/cu31924070153519 |access-date= 11 July 2018}}

{{Imperial units}}
{{United States Customary Units}}

[[Category:Customary units of measurement in the United States]]
[[Category:Imperial units]]
[[Category:Units of length]]

fy:Tomme (lingtemaat)

{{Sourceattribution|Inch}}