Theimperial gallon (imp gal) is defined as4.54609litres, and is or was used in the United Kingdom and its former colonies, including Ireland, Canada, Australia, New Zealand, India, South Africa, Malaysia and someCaribbean countries, while theUS gallon (US gal) is defined as 231 cubic inches (3.785411784 L),[1] and is used in the United States and someLatin American and Caribbean countries.
There are fourgills in a pint, twopints in a quart, and fourquarts (quarter 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 isgal,[2] not to be confused with thegal (symbol: Gal), aCGS unit ofacceleration.
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.[3] It is used in Britain and some otherCommonwealth countries, and until 1976 was defined as the volume of water at 62 °F (16.7 °C)[4][5] whose mass is 10 pounds (4.5359237 kg).
There are fourimperial quarts in a gallon, twoimperial pints in a quart, and 20 imperialfluid ounces in an imperial pint,[3] making an imperial fluid ounce1/160 of an imperial gallon.
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 231cubic inches, i.e. 3.785411784 litres.[6][7]
A US gallon contains 3.785411784 kg (8.3454 lb) of water at 3.983 °C (39.169 °F), making it 83.26742% of an imperial gallon. There are fourquarts in a gallon, twopints in a quart and 16 US fluid ounces in aUS pint, making the US fluid ounce1/128 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[8] and alcoholic beverages[9] are both referenced to 60 °F (15.6 °C) in government regulations.
The United Arab Emirates ceased selling petrol by the imperial gallon in 2010 and switched to the litre, with Guyana following suit in 2013.[23][24][25] In 2014,Myanmar switched from the imperial gallon to the litre.[26]
Antigua and Barbuda has proposed switching to selling petrol by litres since 2015.[27][16]
In theEuropean Union the gallon was removed from the list of legally defined primary units of measure catalogue in theEU 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.[28]
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.[29][30][31][32] 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.[33][34][35]
Both the imperial gallon and the US gallon are used in theTurks 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),[43] andthe Bahamas.[44][45]
Both the US gallon and imperial gallon are divided into four quarts (quarter gallons), which in turn are divided into twopints, which in turn are divided into twocups (not in customary use outside the US), which in turn are further divided into twogills. 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 is1/20 of an imperial pint or1/160 of an imperial gallon, while a US fluid ounce is1/16 of a US pint or1/128 of a US gallon.
As an imperial fluid ounce is 96.076% of a US fluid ounce, this means that one imperial gallon, quart, pint, cup and gill are all equal to 1.20095 of their US counterparts.
Historically, a common bottle size forliquor in the US was the "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".[47][48]
An Americanmilk bottle with a volume of one US gallon
The term derives most immediately fromgalun,galon inOld Norman French,[51] but the usage was common in several languages, for examplejale inOld French andgęllet (bowl) in Old English. This suggests a common origin inRomance Latin, but the ultimate source of the word is unknown.[52]
The gallon originated as the basis of systems for measuringwine andbeer in England. The sizes of gallon used in these two systems were different from each other: the first was based on thewine 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:
Thecorn gallon (orWinchester gallon) of about268.8025 cubic inches (≈4.404884 L),
Thecorn ordry 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 of18+1/2 inches in diameter and 8 inches in depth, which made the bushel8 in × (9+1/4 in)2 ×π ≈ 2150.42017 cubic inches. The bushel was later redefined to be 2150.42 cubic inches exactly, thus making its gallon exactly268.8025 in3 (4.40488377086 L); in previous centuries, there had been a corn gallon of between 271 and 272 cubic inches.
Thewine gallon was legally adopted as the standard US gallon in 1836. Some sources relate this to the volume occupied by eight medievalmerchant 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.6 in × (3+1/2 in)2 ×π ≈ 230.90706 cubic inches. It was redefined in 1706 during the reign ofQueen Anne as being exactly231 in3, the earlier definition withπ being approximated as22/7.
Although the wine gallon had been used for centuries for import duty purposes, there was no legal standard of it in theExchequer, and a smaller gallon224 in3 (3.670702336 L) was actually in use, which required this statute to resolve these issues:231 in3 remains the definition of a gallon in the US today.
In 1824, Britain adopted theimperial 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 30inches of mercury (100kilopascals; 15pounds per square inch) and at a temperature of 62 °F (16.7 °C), which was calculated as277.274 in3 (or4.543706784 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 as277.463 in3 (or4.546803939 L to ten significant figures).
In 1963, the definition was again refined as the space occupied by 10 pounds of distilled water of density0.998859 g/mL weighed in air of density0.001217 g/mL against weights of density8.136 g/mL (the original "brass" was refined as the densities ofbrass alloys vary depending on metallurgical composition), which was calculated as4.546091879 L (≈ 277.4195 in3) to ten significant figures.[4]
The definition of exactly4.54609 cubic decimetres (also4.54609 L or ≈ 277.4194 in3) came after the litre was redefined in 1964. This was adopted shortly afterwards in Canada, and adopted in 1976 in the United Kingdom.[4]
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 thelitre.
^Thompson, E Ambler; Taylor, Barry N (2008).Guide for the use of the International System of Units (SI). National Institute of Standards and Technology.doi:10.6028/nist.sp.811e2008.
^abcBS 350:Part 1:1974 Conversion factors and tables Part 1. Basis of tables. Conversion factors (AMD 4153 ed.). British Standards Institution. 1983. p. Foreword.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 dm3 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)...
^BS 350:Part 1:1974 Conversion factors and tables Part 1. Basis of tables. Conversion factors (prior to Amendment No.1 1983 ed.). British Standards Institution. 1974. p. 10.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.
^"Anguilla Renewable Energy Integration Project Final Report"(PDF).Anguilla RE Integration Final Report. Government of Anguilla Ministry of Infrastructure, Communications, Utilities, and Housing (MICUH). 19 October 2012. p. 104.Archived(PDF) from the original on 14 October 2013. Retrieved13 October 2013.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.
