Editing Gallon

{{short description|Units of volume}}
{{other uses}} 
{{Use dmy dates|date=January 2020}}
{{Infobox unit
| name         = gallon
| image        = GasCan.jpg
| caption      = A one-US-gallon gas can showing "U.S. Gallon" marking (for American use), imperial gallons (for British use), and litres (for Canadian use)
| standard     = 
| quantity     = Volume
| symbol       = gal
| extralabel   = 
| extradata    = 
| units_imp1   = [[International System of Units#Non-SI units accepted for use with SI|Non-SI units accepted for use with SI]]
| inunits_imp1 = {{convert|1|impgal|L|sigfig=6|disp=out}}
| units_imp2   = US customary units
| inunits_imp2 = ≈{{thinsp}}{{convert|1|impgal|usgal|sigfig=7|disp=out}}
| units_imp3   = US customary units
| inunits_imp3 = ≈{{thinsp}}{{convert|1|impgal|in3|sigfig=7|disp=out}}
| units_us1    = Non-SI units
| inunits_us1  = {{thinsp}}{{convert|1|usgal|L|sigfig=10|disp=out}}
| units_us2    = Imperial units
| inunits_us2  = ≈{{thinsp}}{{convert|1|usgal|impgal|sigfig=7|disp=out}}
| units_us3    = Imperial units
| inunits_us3  = {{convert|1|usgal|in3|sigfig=3|disp=out}}
}}

The '''gallon''' is a [[unit of measurement|unit]] of volume in [[British imperial units]] and [[United States customary units]].

The '''imperial gallon''' (imp gal) is defined as {{val|4.54609|u=[[litre]]s}}, and is or was used in the United Kingdom and its former colonies, including Ireland, Canada, Australia, New Zealand, India, South Africa, Malaysia and some [[Caribbean]] countries, while the '''US gallon''' (US gal) is defined as {{convert|231|in3|L| |sigfig=10|comma=gaps}},<ref>{{cite book |last1=Thompson |first1=E Ambler |last2=Taylor |first2=Barry N |date=2008 |title=Guide for the use of the International System of Units (SI) |publisher=National Institute of Standards and Technology |doi=10.6028/nist.sp.811e2008 |doi-access=free}}</ref> and is used in the United States and some [[Latin America]]n and Caribbean countries.

There are four [[gill (unit)|gill]]s in a pint, two [[pint]]s in a quart, and four [[quart]]s (''quart''er gallons) in a gallon, with the imperial gill being divided into five imperial fluid ounces and the US gill being divided into four US fluid ounces: this, and a slight difference in the sizes of the imperial fluid ounce and the US fluid ounce, give different sizes for the imperial gallon and US gallon.

The IEEE standard symbol for both the imperial and US gallons is '''gal''',<ref>IEEE Std 260.1–2014</ref> not to be confused with the [[gal (unit)|gal]] (symbol: Gal), a [[Centimetre–gram–second system of units|CGS]] unit of [[acceleration]].

==Definitions==
The gallon currently has two definitions, in the imperial system and in the US customary system. 

Historically, there were many definitions and redefinitions: see {{section link|Gallon|Sizes of gallons|nopage=y}} for details.

===Imperial gallon===
[[File:Shell petrol station in the UK.jpg|thumb|A Shell petrol station selling 2* and 4* (leaded petrol) by the gallon in the UK, circa 1980]]
The British imperial gallon (frequently called simply "gallon") is defined as exactly 4.54609 litres.<ref name=W&M1985>{{cite web|title=Weights and Measures Act 1985, chapter 72, schedule 1|url=https://www.legislation.gov.uk/ukpga/1985/72/schedule/1|website=legislation.gov.uk|publisher=The National Archives on behalf of HM Government|access-date=7 June 2019}}</ref> It is used in Britain and some other [[Commonwealth of Nations|Commonwealth countries]], and until 1976 was defined as the volume of water at {{cvt|62|F|1|abbr=out}}<ref name=BS350Foreword>{{cite book|title=BS 350:Part 1:1974 Conversion factors and tables Part 1. Basis of tables. Conversion factors|date=1983|publisher=British Standards Institution|page=Foreword|edition=AMD 4153|quote=Before that date (November 1976) the definition in the Weights and Measures Act 1963 was such that the gallon could be calculated to be 4.546 091 879 dm<sup>3</sup> to ten significant figures... The return, in November 1976, by precise definition to what had earlier been used as an approximation for the value of the gallon (i.e. 4.546 09 dm3)...}}</ref><ref>{{cite book|title=BS 350:Part 1:1974 Conversion factors and tables Part 1. Basis of tables. Conversion factors|date=1974|publisher=British Standards Institution|page=10|edition=prior to Amendment No.1 1983|quote=the UK gallon (imp gal), defined in Schedule 1 of the Weights and Measures Act 1963, as the space occupied by 10 pounds of distilled water under certain conditions specified in the schedule.}}</ref> whose mass is {{convert|10|lb|sigfig=8}}. 

There are four [[imperial quart]]s in a gallon, two [[imperial pint]]s in a quart, and 20 imperial [[fluid ounce]]s in an imperial pint,<ref name=W&M1985/> making an imperial fluid ounce {{sfrac|160}} of an imperial gallon.

===US gallon===
{{See also|United States customary units#Fluid volume|l1=Fluid volume}}
[[File:US petrol pump.jpg|thumb|A fuel station in the United States displaying fuel prices per US gallon]]
The US gallon (frequently called simply "gallon") is legally defined as exactly 231 [[cubic inch]]es, i.e. {{convert|231|in3|L|9|disp=out|abbr=off|comma=off}}.<ref>{{cite journal|url=https://www.nist.gov/pml/wmd/h44-12.cfm|title=NIST Handbook 44 – 2012 Edition Appendix C "General Tables of Units of Measurement"|journal=NIST |date=26 October 2012|page=C-5}}</ref><ref>{{cite book|title=Uniform Laws and Regulations in the areas of legal metrology and engine fuel quality|year=2011|publisher=US Department of Commerce, National Institute of Standards and Technology|pages=9–13, 69|url=https://www.nist.gov/pml/wmd/pubs/upload/HB130-11-final.pdf |archive-url=https://web.archive.org/web/20120927010401/http://www.nist.gov/pml/wmd/pubs/upload/HB130-11-final.pdf |archive-date=2012-09-27 |url-status=live}}</ref>

A US gallon contains {{cvt|3.785411784|kg|lb|sigfig=5}} of water at {{cvt|3.983|C|F}}, making it {{#expr: 3.785411784/4.54609*100 round 5}}% of an imperial gallon. There are four [[quart]]s in a gallon, two [[pint]]s in a quart and 16 US fluid ounces in a [[US pint]], making the US fluid ounce {{sfrac|128}} of a US gallon.

In order to overcome the effects of expansion and contraction with temperature when using a gallon to specify a quantity of material for purposes of trade, it is common to define the temperature at which the material will occupy the specified volume. For example, the volume of petroleum products<ref>{{Cite web|url=http://www.nh.gov/agric/news/documents/weights-fuel-temperature.pdf|archiveurl=https://web.archive.org/web/20120413013031/http://www.nh.gov/agric/news/documents/weights-fuel-temperature.pdf|url-status=dead|title=State of New Hampshire Dept of Weights and Measure|archivedate=13 April 2012}}</ref> and alcoholic beverages<ref>{{Cite web|url=http://www.gpo.gov/fdsys/pkg/CFR-2011-title27-vol1/pdf/CFR-2011-title27-vol1-sec5-21.pdf |archive-url=https://web.archive.org/web/20131019135845/http://www.gpo.gov/fdsys/pkg/CFR-2011-title27-vol1/pdf/CFR-2011-title27-vol1-sec5-21.pdf |archive-date=2013-10-19 |url-status=live|title=27 CFR section 5.21|website=Gpo.gov|access-date=29 January 2022}}</ref> are both referenced to {{cvt|60|F|1|lk=off}} in government regulations.

==Worldwide usage==
[[File:Gasoline unit.svg|thumb|[[Petrol]] units used in the world:
{{legend|#ff0000|[[Litre]]}}{{legend|#00ff00|US gallon}}{{legend|#0000ff|Imperial gallon}}{{legend|#e0e0e0|No data}}]]

===Imperial gallon===
As of 2021, the imperial gallon continues to be used as the standard petrol unit on 10 [[List of Caribbean islands|Caribbean island groups]], consisting of:

* four [[British Overseas Territories]] ([[Anguilla]],<ref>{{cite web|date=19 October 2012|title=Anguilla Renewable Energy Integration Project Final Report|url=http://cdkn.org/wp-content/uploads/2012/08/Anguilla-RE-Integration-Final-Report-121019.pdf |archive-url=https://web.archive.org/web/20131014175039/http://cdkn.org/wp-content/uploads/2012/08/Anguilla-RE-Integration-Final-Report-121019.pdf |archive-date=2013-10-14 |url-status=live|access-date=13 October 2013|work=Anguilla RE Integration Final Report|publisher=Government of Anguilla Ministry of Infrastructure, Communications, Utilities, and Housing (MICUH)|page=104|quote=In 2008—the most recent year where WTI crude oil averaged US$100 per barrel—ANGLEC paid an average of about US$4 per imperial gallon (imp gal) for diesel.}}</ref> the [[British Virgin Islands]],<ref>{{cite web|last=Walker|first=William|date=5 September 2012|title=Biwater project draws scrutiny|url=http://www.bvibeacon.com/2/index.php?option=com_content&view=article&id=2487:biwater-project-draws-scrutiny&catid=491:commentary-sept-6-2012|archive-url=https://web.archive.org/web/20131002140602/http://www.bvibeacon.com/2/index.php?option=com_content&view=article&id=2487:biwater-project-draws-scrutiny&catid=491:commentary-sept-6-2012|archive-date=2013-10-02|access-date=2013-09-30}}</ref><ref>{{Cite web |title=Statement By Premier Fahie In One-On-One Press Conference – Update On Current Matters {{!}} Government of the Virgin Islands |url=https://bvi.gov.vg/media-centre/statement-premier-fahie-one-one-one-press-conference-update-current-matters |access-date=2022-12-16 |website=bvi.gov.vg}}</ref> the [[Cayman Islands]],<ref>{{cite news|last=Wilson|first=Stuart|date=9 May 2013|title=Fuel costs driven by factors|work=Cayman Compass|url=http://www.compasscayman.com/caycompass/2013/05/09/Fuel-costs-driven-by-factors/|access-date=30 September 2013|archive-date=22 December 2015|archive-url=https://web.archive.org/web/20151222131450/http://www.compasscayman.com/caycompass/2013/05/09/Fuel-costs-driven-by-factors/|url-status=dead}}</ref> and [[Montserrat]])<ref>{{cite news|date=30 November 2012|title=Petrol price increases, diesel decreases, cost of living goes up|work=The Montserrat Reporter|url=http://www.themontserratreporter.com/gasoline-price-increases-diesel-decreases-cost-of-living-goes-up/|access-date=30 September 2013|archive-date=3 October 2022|archive-url=https://web.archive.org/web/20221003154555/http://www.themontserratreporter.com/gasoline-price-increases-diesel-decreases-cost-of-living-goes-up/|url-status=dead}}</ref><ref>{{cite news|author=Htin Lynn Aung|date=22 September 2017|title=Fuel prices on the rise|work=[[The Myanmar Times]]|url=https://www.mmtimes.com/news/fuel-prices-rise.html|access-date=18 January 2018|archive-date=18 January 2018|archive-url=https://web.archive.org/web/20180118064732/https://www.mmtimes.com/news/fuel-prices-rise.html|url-status=dead}}</ref> and 
* six countries ([[Antigua and Barbuda]],<ref name=":0">{{Cite web|title=Government of Antigua and Barbuda|url=https://ab.gov.ag/media_page.php?page=17|access-date=2021-05-05|website=ab.gov.ag}}</ref> [[Dominica]],<ref>{{cite web|date=10 May 2011|title=Photo of the day: Up and up it goes|url=http://dominicanewsonline.com/news/homepage/features/photo-of-the-day/photo-of-the-day-up-and-up-it-goes/|work=Dominica News Online|access-date=30 September 2013|archive-date=21 February 2018|archive-url=https://web.archive.org/web/20180221103436/http://dominicanewsonline.com/news/homepage/features/photo-of-the-day/photo-of-the-day-up-and-up-it-goes/|url-status=dead}}</ref> [[Grenada]],<ref>{{cite book|url=https://books.google.com/books?id=p1g4z5GpiBoC&pg=PA17|title=Grenada: Third Review Under the Three-Year Arrangement Under the Poverty Reduction and Growth Facility, Requests for Modification of Quantitative Performance Criterion and Augmentation, and Financing Assurances Review|publisher=[[International Monetary Fund]]|year=2009|page=17|access-date=13 October 2013}}</ref><ref>{{Cite web|date=2020-05-20|title=Authorities in Dominica reduce fuel prices|url=https://antiguaobserver.com/authorities-in-dominica-reduce-fuel-prices/|access-date=2021-05-05|website=Antigua Observer Newspaper|language=en-CA}}</ref> [[Saint Christopher and Nevis]],<ref>{{cite web|last=Thomas|first=Steve|date=5 September 2008|title=Gas prices capped under $18 – lowest since July|url=http://www.thestkittsnevisobserver.com/2008/09/05/news21.html|url-status=dead|archive-url=https://web.archive.org/web/20120524184004/http://thestkittsnevisobserver.com/2008/09/05/news21.html|archive-date=24 May 2012|work=[[The St. Kitts-Nevis Observer]]|df=dmy}}</ref> [[Saint Lucia]],<ref>{{cite web|title=Rotary has a new President – St. Lucia Voice News|url=http://www.thevoiceslu.com/local_news/2013/july/09_07_13/Rotary.htm|url-status=dead|archive-url=https://web.archive.org/web/20151222145630/http://www.thevoiceslu.com/local_news/2013/july/09_07_13/Rotary.htm|archive-date=22 December 2015|df=dmy}}</ref> and [[Saint Vincent and the Grenadines]]<ref>{{cite web|title=Another service station sues SOL over fuel volume discrepancy|url=http://www.iwnsvg.com/2012/04/26/another-service-station-sues-sol-over-fuel-volume-discrepancy/|work=iWitness News|date=26 April 2012 }}</ref>).

All 12 of the Caribbean islands use [[miles per hour]] for speed limits signage, and [[Left- and right-hand traffic|drive on the left side of the road]].

The United Arab Emirates ceased selling petrol by the imperial gallon in 2010 and switched to the litre, with Guyana following suit in 2013.<ref>{{Cite web|title=Petrol stations in UAE go the metric route|url=https://gulfnews.com/business/energy/petrol-stations-in-uae-go-the-metric-route-1.559595|access-date=2021-05-05|website=gulfnews.com|language=en}}</ref><ref>{{Cite web|date=2009-12-15|title=End of road for imperial gallon as UAE switches to buying fuel by litre|url=https://www.thenationalnews.com/uae/end-of-road-for-imperial-gallon-as-uae-switches-to-buying-fuel-by-litre-1.548176|access-date=2021-05-05|website=The National|language=en}}</ref><ref>{{Cite web|date=2013-06-28|title=Gas prices at Guyoil stations remain below $1,000 mark {{!}} Caribbean Mi…|url=http://www.caribmillers.com/content/gas-prices-guyoil-stations-remain-below-1000-mark|access-date=2021-05-05|website=Archive.is|archive-date=28 June 2013|archive-url=https://archive.today/20130628202944/http://www.caribmillers.com/content/gas-prices-guyoil-stations-remain-below-1000-mark|url-status=dead}}</ref> In 2014, [[Myanmar]] switched from the imperial gallon to the litre.<ref>{{Cite web|url=http://www.oilseedcrops.org/2014/04/05/petrol-prices-march-2014-in-yangon-myanmar/|title=Petrol Prices March 2014 in Yangon, Myanmar|website=Oilseedcrops.org|date=5 April 2014}}</ref>

Antigua and Barbuda has proposed switching to selling petrol by litres since 2015.<ref>{{cite web|year=2013|title=The Re-Launch of Antigua And Barbuda's Metrication Programme|url=http://www.cbji.com/makingnews.aspx?id=692|url-status=dead|archive-url=https://web.archive.org/web/20120116182201/http://www.cbji.com/makingnews.aspx?id=692|archive-date=16 January 2012|access-date=20 May 2013|publisher=Diversity Global Magazine}}</ref><ref name=":0"/>

In the [[European Union]] the gallon was removed from the list of legally defined primary units of measure catalogue in the [[European units of measurement directives|EU directive 80/181/EEC]] for trading and official purposes, effective from 31 December 1994. Under the directive the gallon could still be used, but only as a supplementary or secondary unit.<ref>{{cite web|author=The Council of the European Communities|date=9 February 2000|title=Council Directive 80/181/EEC of 20 December 1979 on the approximation of the laws of the Member States relating to Unit of measurement and on the repeal of Directive 71/354/EEC|url=http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=CONSLEG:1980L0181:20000209:EN:PDF|access-date=7 February 2009|quote=The legal units of measurement ... for economic, public health, public safety or administrative purposes ... litre}}</ref>  

As a result of the EU directive Ireland and the United Kingdom passed legislation to replace the gallon with the litre as a primary unit of measure in trade and in the conduct of public business, effective from 31 December 1993, and 30 September 1995 respectively.<ref>{{cite web|date=13 July 1995|title=The Units of Measurement Regulations 1995 (Article 4)|url=http://www.opsi.gov.uk/si/si1995/Uksi_19951804_en_1.htm|access-date=1 December 2011}}</ref><ref>{{cite web|year=1995|title=Units of Measurement Directive|url=http://www.lacors.gov.uk/lacors/ContentDetails.aspx?id=2515|url-status=dead|archive-url=https://web.archive.org/web/20120424231200/http://www.lacors.gov.uk/lacors/ContentDetails.aspx?id=2515|archive-date=24 April 2012|access-date=26 November 2011|publisher=[[LACORS]]|df=dmy}}</ref><ref>{{cite web|year=1995|title=Guidance Note on the use of Metric Units of Measurement by the Public Sector|url=http://www.nmo.bis.gov.uk/Docs/Legislation/Units%20of%20Measurement/Gnotes%20for%20public%20sector%20on%20use%20of%20metric.pdf|url-status=dead|archive-url=http://webarchive.nationalarchives.gov.uk/20110207120946/http://www.nmo.bis.gov.uk/Docs/Legislation/Units%20of%20Measurement/Gnotes%20for%20public%20sector%20on%20use%20of%20metric.pdf|archive-date=7 February 2011|access-date=20 March 2017|publisher=[[Department of Trade and Industry (United Kingdom)|Department of Trade and Industry]]|df=dmy}}</ref><ref>{{cite web|date=9 September 1992|title=S.I. No. 255/1992 — European Communities (Units of Measurement) Regulations, 1992.|url=http://www.irishstatutebook.ie/1992/en/si/0255.html|access-date=1 December 2011|work=Irish Statute Book|publisher=Office of the Attorney General}}</ref> Though the gallon has ceased to be a primary unit of trade, it can still be legally used in both the UK and Ireland as a supplementary unit. However, barrels and large containers of beer, oil and other fluids are commonly measured in multiples of an imperial gallon. 

Miles per imperial gallon is used as the primary fuel economy unit in the United Kingdom and as a supplementary unit in Canada on official documentation.<ref>{{cite web|date=5 November 2008|title=FAQs – Fuel Consumption Program|url=http://www.tc.gc.ca/eng/programs/environment-fcp-faq-139.htm|url-status=live|archive-url=https://web.archive.org/web/20100520103922/http://www.tc.gc.ca/eng/programs/environment-fcp-faq-139.htm|archive-date=20 May 2010|access-date=31 July 2018|publisher=Transport Canada|df=dmy-all}}</ref><ref>{{cite book|url=http://www.legislation.gov.uk/uksi/2001/3523/pdfs/uksi_20013523_en.pdf |archive-url=https://web.archive.org/web/20101218054954/http://www.legislation.gov.uk/uksi/2001/3523/pdfs/uksi_20013523_en.pdf |archive-date=2010-12-18 |url-status=live|title=Statutory Instrument 2001/3523 Environmental Protection – The Passenger Car (Fuel Consumption and CO<sub>2</sub> Emissions Information) Regulations 2001|date=30 October 2001|publisher=The Stationery Office|isbn=0-11-038743-0|access-date=1 December 2011}}</ref><ref>{{Cite web|last=Canada|first=Natural Resources|date=2018-04-30|title=energuide-vehicles|url=https://www.nrcan.gc.ca/energy-efficiency/energuide-canada/energuide-vehicles/21010|access-date=2021-02-21|website=Nrcan.gc.ca}}</ref>

===US gallon===
Other than the United States, [[petrol]] is sold by the US gallon in seven other countries and four US territories:
* the Caribbean country of [[Haiti]],
* the Central American countries of [[Belize]] and [[Guatemala]],
* the Pacific Ocean countries of [[Marshall Islands]], [[Federated States of Micronesia]], and [[Palau]], which are [[Associated state|associated]] countries of the United States,
* the African country of [[Liberia]], a former [[protectorate]] of the United States,<ref>{{Cite web |date=June 10, 2020 |title=Liberia: Senate Imposes US$0.30 Tax on Each Gallon of Petroleum Product. |url=https://frontpageafricaonline.com/news/liberia-senate-imposes-us0-30-tax-on-each-gallon-of-petroleum-product-approves-fy2019-2020-recast-budget-at-us518m/}}</ref> and
* the US territories of [[American Samoa]],<ref>{{cite web|url=http://www.samoanews.com/?q=node/76479 |title=Gasoline goes up eleven cents per gallon tomorrow |work=Samoa News |url-status=dead |archive-url=https://web.archive.org/web/20160304201034/http://www.samoanews.com/?q=node%2F76479 |archive-date=4 March 2016 }}</ref> the [[Northern Mariana Islands]],<ref>{{cite web|url=http://www.saipantribune.com/newsstory.aspx?newsID%3D57572%26cat%3D1 |title=Shell pump prices up 8 cents |work=Saipan Tribune |first=Liberty |last=Dones |date= 17 May 2006 |access-date=30 September 2013 |url-status=dead |archive-url=https://web.archive.org/web/20061030231121/http://www.saipantribune.com/newsstory.aspx?newsID=57572&cat=1 |archive-date=30 October 2006 |df=dmy-all }}</ref> [[Guam]],<ref>{{cite web|url=http://www.guampdn.com/article/20130808/NEWS01/130808005/UPDATE-Gas-prices-down-10-cents-4-73-gallon-unleaded |archive-url=https://archive.today/20130930053836/http://www.guampdn.com/article/20130808/NEWS01/130808005/UPDATE-Gas-prices-down-10-cents-4-73-gallon-unleaded |url-status=dead |archive-date=2013-09-30 |title=UPDATE: Gas prices down 10 cents to $4.73 for a gallon of unleaded |work=Pacific Daily News }}</ref> and the [[United States Virgin Islands|US Virgin Islands]].<ref>{{cite news|url=http://virginislandsdailynews.com/news/7-cent-per-gallon-wapa-tax-goes-into-effect-1.1344082 |archive-url=https://archive.today/20130930053844/http://virginislandsdailynews.com/news/7-cent-per-gallon-wapa-tax-goes-into-effect-1.1344082 |url-status=dead |archive-date=30 September 2013 |title=7-cent-per-gallon WAPA tax goes into effect |first=Joy |last=Blackburn |date=16 July 2012 |newspaper=Virgin Islands Daily News |df=dmy }}</ref> [[Puerto Rico]] ceased selling petrol by the US gallon in 1980.<ref>{{citation|last1=Pesquera de Busquets|first1=Carmen T|last2=Barcelo|first2=Carlos Romero|date=14 June 1979|title=Order to establish the price of half 1/2 galon [sic] of gasoline as transitory measure and that the litter [sic] should be the final metric measurement for the sale of gasoline in Puerto Rico|url=http://app.estado.gobierno.pr/ReglamentosOnLine/Reglamentos/2516ING.pdf |archive-url=https://web.archive.org/web/20131114144310/http://app.estado.gobierno.pr/ReglamentosOnLine/Reglamentos/2516ING.pdf |archive-date=2013-11-14 |url-status=live|journal=|location=San Juan, Puerto Rico|publisher=Departamento de Asuntos del Consumidor|access-date=21 May 2013}}</ref>

The latest country to cease using the US gallon is [[El Salvador]] in June 2021.<ref>{{Cite web|date=2020-12-16|title=La gasolina se venderá en litros y otros productos se pesarán en kilogramos, según nuevo sistema métrico a implementarse en 2021 {{!}} Noticias de El Salvador|url=https://www.elsalvador.com/noticias/negocios/gasolina-se-vendera-en-litros-2021/786601/2020/|access-date=2021-05-05|website=Noticias de El Salvador - elsalvador.com|language=es}}</ref>

===Both imperial and US gallon===
Both the imperial gallon and the US gallon are used in the [[Turks and Caicos Islands]], due to an increase in tax duties which was disguised by levying the same duty on the US gallon (3.79 L) as was previously levied on the Imperial gallon (4.55 L), <ref>{{cite web|url=http://www.thebahamasweekly.com/publish/TCI/New_Measures_To_Improve_Tci_Finances_And_Prioritise_Spending_printer.shtml|website=TheBahamasWeekly.com|title=New Measures to Improve TCI Finances And Prioritise Spending}}</ref> and [[the Bahamas]].<ref>{{cite web|url=http://laws.bahamas.gov.bs/cms/images/LEGISLATION/SUBORDINATE/2014/2014-0005/ProtectionofRevenueImpostsVariationAmendmentOrder2014_1.pdf |archive-url=https://web.archive.org/web/20210602214208/http://laws.bahamas.gov.bs/cms/images/LEGISLATION/SUBORDINATE/2014/2014-0005/ProtectionofRevenueImpostsVariationAmendmentOrder2014_1.pdf |archive-date=2021-06-02 |url-status=live|title=OFFICIAL GAZETTE THE BAHAMAS PUBLISHED BY AUTHORITY : NASSAU |date=12 February 2014 |website=Laws.bahamas.gov.bs|access-date=29 January 2022}}</ref><ref>{{cite web |title=Weights and Measures |work=Statute Law of the Bahamas |url=https://www.bbsq.bs/en/files/acts-and-regulations/1-weights-and-measures-act/file |access-date=1 June 2021 |url-status=dead |archive-url=https://web.archive.org/web/20210602215910/https://www.bbsq.bs/en/files/acts-and-regulations/1-weights-and-measures-act/file |archive-date=2 June 2021}}</ref>

===Legacy===
In some parts of the Middle East, such as the [[United Arab Emirates]] and [[Bahrain]], 18.9-litre water cooler bottles are marketed as five-gallon bottles.<ref>{{Cite web |title=AL MANHAL DRINKING WATER 5 GALLON ( 18.9 LITERS ) BOTTLE |url=https://www.buyonline.nestlewaters.bh/al-manhal-drinking-water#}}</ref>

==Relationship to other units==
Both the US gallon and imperial gallon are divided into four quarts (''quart''er gallons), which in turn are divided into two [[pint]]s, which in turn are divided into two [[cup (unit)|cups]] (not in customary use outside the US), which in turn are further divided into two [[gill (unit)|gills]]. Thus, both gallons are equal to four quarts, eight pints, sixteen cups, or thirty-two gills.

There is a difference in that the imperial gill is further divided into five fluid ounces, whereas the US gill is divided into four fluid ounces: this means that an imperial fluid ounce is {{sfrac|20}} of an imperial pint or {{sfrac|160}} of an imperial gallon, while a US fluid ounce is {{sfrac|16}} of a US pint or {{sfrac|128}} of a US gallon. 

As an imperial fluid ounce is {{#expr: 28.4130625/29.5735295625*100 round 3}}% of a US fluid ounce, this means that one imperial gallon, quart, pint, cup and gill are all equal to {{#expr: 28.4130625/29.5735295625*5/4 round 5}} of their US counterparts.

Historically, a common bottle size for [[liquor]] in the US was the "[[Fifth (unit)|fifth]]", i.e. one-fifth of a US gallon (or 0.08% more than a "reputed quart", one-sixth of an imperial gallon). While spirit sales in the US were switched to metric measures in 1976, a 750 mL bottle is still sometimes known as a "fifth".<ref>E. Frank Henriques, ''The Signet Encyclopedia of Wine'', p. 298</ref><ref>{{cite news|url=https://www.nytimes.com/1976/10/11/archives/liquor-industry-converts-to-metric-system-liquor-industry-is.html|date=11 October 1976|title=Liquor Industry Converts to Metric System|first=Rona|last=Cherry|work=[[The New York Times]]}}</ref>

===US dry gallon===
The [[dry gallon|US dry gallon]] was defined as one-eighth of a US ''[[Winchester measure|Winchester]]'' [[bushel]] of exactly {{val|2150.42|fmt=commas}} cubic inches, i.e. 268.8025 cubic inches or {{val|4.40488377086|u=L}}.<ref>{{Cite web|url=https://keisan.casio.com/exec/system/1344320515|title=US Dry Conversion Calculator|website=High accuracy calculation for life or science.|language=en|access-date=2019-10-24|archive-date=24 October 2019|archive-url=https://web.archive.org/web/20191024225749/https://keisan.casio.com/exec/system/1344320515|url-status=dead}}</ref> 

The US dry gallon is no longer used, and is no longer included in the relevant statute, which goes from the [[Quart#US dry quart|dry quart]] to the [[peck]].<ref name="15USC205">[https://www.law.cornell.edu/uscode/search/display.html?terms=unit%20measure&url=/uscode/html/uscode15/usc_sec_15_00000205----000-notes.html ''Authorized tables'']'', US Code'', Title 15, ch. 6, subchapter I, sec. 205, accessed 19 July 2008.</ref>

==History==
[[File:Bottle of milk.jpg|thumb|An American [[Plastic milk container|milk bottle]] with a volume of one US gallon]]
The term derives most immediately from ''galun'', ''galon'' in [[Old Norman|Old Norman French]],<ref>{{cite EB1911 |wstitle=Gallon |volume=11 |page=421}}</ref> but the usage was common in several languages, for example {{lang|fro|jale}} in [[Old French]] and {{lang|ang|gęllet}} (bowl) in Old English. This suggests a common origin in [[Vulgar Latin|Romance Latin]], but the ultimate source of the word is unknown.<ref>{{cite book|title=[[Oxford English Dictionary]]|edition=2|year=1989|publisher=Oxford University Press|location=Oxford, England|chapter=gallon, n.}}</ref>

The gallon originated as the basis of systems for measuring [[English wine cask units|wine]] and [[English brewery cask units|beer]] in England. The sizes of gallon used in these two systems were different from each other: the first was based on the [[wine gallon]] (equal in size to the US gallon), and the second one the ale gallon (1.65% larger than the imperial gallon).

By the end of the 18th century, there were three definitions of the gallon in common use:
* The ''corn gallon'' (or ''Winchester gallon'') of about {{nowrap|268.8025 cubic inches}} (≈{{val|4.404884|u=L}}),
* The ''[[wine gallon]]'' (or ''[[Queen Anne's gallon]]'') of {{nowrap|231 cubic inches}}<ref>{{cite encyclopedia|url=http://www.sizes.com/units/gallon_english_wine.htm|title=English wine gallon|encyclopedia=Sizes.com|access-date=2010-06-17}}</ref> ({{val|3.785411784|u=L}}), and
* The ''ale gallon'' of {{nowrap|282 cubic inches}} ({{val|4.621152048|u=L}}).

The ''corn'' or ''dry gallon'' was used in the United States for grain and other dry commodities. It was one-eighth of the (Winchester) bushel, originally defined as a cylindrical measure of {{sfrac|18|1|2}} inches in diameter and 8&nbsp;inches in depth, which made the bushel {{nowrap|8&nbsp;in × ({{sfrac|9|1|4}} in)<sup>2</sup> × [[Pi|{{pi}}]] ≈ 2150.42017 cubic inches}}. The bushel was later redefined to be 2150.42&nbsp;cubic inches exactly, thus making its gallon exactly {{val|268.8025|u=in3}} ({{val|4.40488377086|u=L}}); in previous centuries, there had been a corn gallon of between 271 and 272 cubic inches.

The ''wine gallon'' was legally adopted as the standard US gallon in 1836. Some sources relate this to the volume occupied by eight medieval [[English unit#Weight|merchant pounds]] of wine: this was at one time defined as the volume of a cylinder 6 inches deep and 7 inches in diameter, i.e. {{nowrap|6&nbsp;in × ({{sfrac|3|1|2}}&nbsp;in)<sup>2</sup> × {{pi}} ≈ 230.90706 cubic inches}}. It was redefined in 1706 during the reign of [[Anne, Queen of Great Britain|Queen Anne]] as being exactly {{val|231|u=in3}}, the earlier definition with {{pi}} being approximated as {{sfrac|22|7}}.

<math display=block> \pi r^2h \approx \frac{22}{7}\times\left ( \frac{7 ~ \mathrm{in}}{2} \right )^2\times6 ~ \mathrm{in} = 231 ~ \mathrm{in}^3.</math>

Although the wine gallon had been used for centuries for import duty purposes, there was no legal standard of it in the [[Exchequer]], and a smaller gallon {{val|224|u=in3}} ({{val|3.670702336|u=L}}) was actually in use, which required this statute to resolve these issues: {{val|231|u=in3}} remains the definition of a gallon in the US today.

In 1824, Britain adopted the ''imperial gallon'', and abolished all other gallons in favour of it. The imperial gallon was defined as the volume of 10 pounds of distilled water weighed in air with brass weights with the barometer standing at {{convert|30|inHg|kPa psi|abbr=off|lk=on}} and at a temperature of {{cvt|62|F|1}}, which was calculated as {{val|277.274|u=in3}} (or {{val|4.543706784|u=L}} to ten significant figures).

This value lasted until 1889, when an Order in Council of November 28 of that year redefined the imperial gallon as {{val|277.463|u=in3}} (or {{val|4.546803939|u=L}} to ten significant figures). 

In 1963, the definition was again refined as the space occupied by 10 pounds of distilled water of density {{val|0.998859|u=g/mL}} weighed in air of density {{val|0.001217|u=g/mL}} against weights of density {{nowrap|8.136 g/mL}} (the original "brass" was refined as the densities of [[brass]] alloys vary depending on metallurgical composition), which was calculated as {{val|4.546091879|u=L}} (≈&nbsp;{{val|277.4195|u=in3}}) to ten significant figures.<ref name=BS350Foreword/>

The definition of exactly {{val|4.54609}} cubic decimetres (also {{val|4.54609|u=L}} or ≈&nbsp;{{val|277.4194|u=in3}}) came after the litre was redefined in 1964. This was adopted shortly afterwards in Canada, and adopted in 1976 in the United Kingdom.<ref name=BS350Foreword/>

===Sizes of gallons===
Historically, gallons of various sizes were used in many parts of Western Europe. In these localities, it has been replaced as the unit of capacity by the [[litre]].

{|class="wikitable" style=text-align:center;
|+ Comparison of gallons
|-
!colspan="2"| Volume
!rowspan="2"| Definition
!rowspan="2"| Inverted<br>volume<br/>(gal/cu ft)
!rowspan="2"| Weight as<br/>water at {{cvt|62|F}}<br/>([[avoirdupois|pounds]]/gal)
!colspan="3"| Cylindrical approximation
|-
! ([[cu in]])
! ([[Litre|dm<sup>3</sup>]])
! Diameter<br/>(in)
! Height<br/>(in)
! Volume [[relative error|rel.<br/>error]] (%)
|-
!colspan=8 |Current gallons
|-
| 231 || {{val|3.785411784}}
| Statute of 5 Queen Anne (1706) <br /> UK wine gallon<br /> '''US gallon''' (legally adopted 1836 US)
| {{sfrac|7|37|77}} || 8.3454
| 7 || 6 || 0.04
|-
| ≈ 277.4194 || {{val|4.54609}}
| '''Imperial gallon''' (adopted 1964 Canada, adopted 1976 UK)
| 6.2288 || 10.0224
| {{sfrac|5|2|3}} || 11 || {{val|0.0002}}
|-
!colspan=8|Historic gallons
|-
| 216 (Roman [[Uncia (unit)|unciae]])|| {{val|3.539605824}}
| Roman [[congius]]
| 8 || 7.8035
| 5 || 11 || 0.01
|-
| 217 || {{val|3.555992888}}
| Irish gallon (1495, re-confirmed 1736)
| 7{{sfrac|209|217}} || 7.8396
| 5.25 || 10 || 0.24
|-
| 224 || {{val|3.670702336}}
| Preserved at the [[Guildhall, London]] (old UK wine gallon)
| 7{{sfrac|5|7}} || 8.0925
| 9 || 3.5 || 0.6
|-
| ≈ 241.3549 || {{val|3.955093289}}
| Jersey gallon (1562)
| 7.1596 || 8.7195
| 6.5 || 7.25 || 0.32
|-
| ≈ 260.3235 || {{val|4.265931799}}
| Guernsey gallon (17th century origins until 1917)
| 6.6379 || 9.4048
| 5 || 13.25 || 0.06
|-
| 264.8 || {{val|4.3392945472}}
| Ancient Rumford quart (1228)
| 6{{sfrac|174|331}} || 9.5665
| 7.5 || 6 || 0.1
|-
| 265.5 || {{val|4.350765492}}
| Exchequer ([[Henry VII of England|Henry VII]], 1497, with rim)
| 6{{sfrac|30|59}} || 9.5918
| 13 || 2 || 0.01
|-
| 266.25 || {{val|4.36305579}}
| Ancient Rumford (1228)
| 6{{sfrac|174|355}} || 9.6189
| 5.5 || 11.25 || 0.39
|-
| 268.4 || {{val|4.398342075}}
| Henry VII (Winchester) corn gallon (1497)
| 6{{sfrac|294|671}} || 9.6966
| 6 || 9.5 || 0.08
|-
| 268.8025 || {{val|4.40488377086}}
| Winchester, statute of 13 & 14 [[William III of England|William III]] <br /> Corn gallon<br /> US dry gallon (no longer used, no longer listed in the relevant statute)
| 6{{sfrac|46074|107521}} || 9.7111
| 18.5 || 1 || {{val|0.00001}}
|-
| 270 || {{val|4.42450728}}
| Elizabeth I corn gallon (1601) 
| 6.4 || 9.7544
| 5 || 13.75 || 0.0072
|-
| 271 || {{val|4.440894344}}
| Exchequer (1601, ''E.'') (old ''corn'' gallon)
| 6{{sfrac|102|271}} || 9.7905
| 4.5 || 17 || 0.23
|-
| 272 || {{val|4.457281408}}
| William III corn gallon (1688)
| 6{{sfrac|6|17}} || 9.8266
| 6 || 9.625 || 0.05
|-
| {{val|277.202578125}} || {{val|4.542536388699375}}
| Statute of 12 Anne (coal gallon), also equal to {{sfrac|1|1|32}} corn gallons
| 6.2337 || 10.0146
| {{sfrac|5|2|3}} || 11 || 0.08
|-
| 277.274 || {{val|4.543706784}} || Imperial gallon, as originally determined in 1824
| 6.2321 || 10.0172
| {{sfrac|5|2|3}} || 11 || 0.05
|-
| ≈ 277.4195 || {{val|4.546091879}}
| Imperial gallon as re-determined in 1895 and defined in 1963
| 6.2288 || 10.0224
| {{sfrac|5|2|3}} || 11 || {{val|0.0001}}
|-
| ≈ 277.463 || {{val|4.54680939}}
| Imperial gallon as defined by Order in Council of 28 November 1889
| 6.2279 || 10.024
| {{sfrac|5|2|3}} || 11 || 0.015
|-
| 278 || {{val|4.555603792}}
| Exchequer (Henry VII, with copper rim)
| 6{{sfrac|30|139}} || 10.0434
| 4.5 || 17.5 || 0.12
|-
| 278.4 || {{val|4.562158618}}
| Exchequer (1601 and 1602 pints)
| 6{{sfrac|6|29}} || 10.0578
| 7 || 7.25 || 0.22
|-
| 280 || {{val|4.58837792}}
| Exchequer (1601 quart)
| 6{{sfrac|6|35}} || 10.1156
| 5 || 14.25 || 0.07
|-
| 282 || {{val|4.621152048}}
| Treasury (beer and ale gallon pre-1824)
| 6{{sfrac|6|47}} || 10.1879
| 6.5 || 8.5 || 0.02
|}

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

==External links==
{{sisterlinks|d=Q178413|c=Category:Gallon|wikt=gallon|s=no|m=no|mw=no|species=no|n=no|q=no|b=no|v=no|voy=Metric and Imperial equivalents}}

{{Imperial units}}
{{United States Customary Units}}
{{systems of measurement|sp=us}}

[[Category:Customary units of measurement in the United States]]
[[Category:Imperial units]]
[[Category:Systems of units]]
[[Category:Units of volume]]
[[Category:Alcohol measurement]]
[[Category:Cooking weights and measures]]