Editing Gallon (section)

==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.