Gallon

The gallon is a unit of volume in British imperial units and United States customary units. Three different versions are in current use:
 * the imperial gallon (imp gal), defined as $4.546 litres$, which is or was used in the United Kingdom, Ireland, Canada, Australia, New Zealand, and some Caribbean countries;
 * the US gallon (US gal), defined as 231 in3), which is used in the United States and some Latin American and Caribbean countries; and
 * the US dry gallon ("usdrygal"), defined as $1/8$ US bushel (exactly $4.405 litres$).

There are two pints in a quart and four quarts in a gallon. Different sizes of pints account for the different sizes of the imperial and US gallons.

The IEEE standard symbol for both US (liquid) and imperial gallon is gal, not to be confused with the gal (symbol: Gal), a CGS unit of acceleration.

Definitions
The gallon currently has one definition in the imperial system, and two definitions (liquid and dry) in the US customary system. Historically, there were many definitions and redefinitions.

English system gallons
There were a number of systems of liquid measurements in the United Kingdom prior to the 19th century.
 * Winchester or corn gallon was 272 in3 (1697 act 8 & 9 Will. 3. c. 22)
 * Henry VII (Winchester) corn gallon from 1497 onwards was 154.80 impfloz
 * Elizabeth I corn gallon from 1601 onwards was 155.70 impfloz
 * William III corn gallon from 1697 onwards was 156.90 impfloz
 * Old English (Elizabethan) ale gallon was 282 in3 (Ale Measures Act 1698 (11 Will. 3. c. 15))
 * London 'Guildhall' gallon (before 1688) was 129.19 impfloz then
 * Old English (Queen Anne) wine gallon was standardized as 231 in3 in the 1706 act 6 Ann. c. 27:
 * Jersey gallon (from 1562 onwards) was 139.20 impfloz
 * Guernsey gallon (17th century origins until 1917) was 150.14 impfloz
 * Irish gallon was 217 in3 (Poynings' Act 1495 (10 Hen. 7. c. 22 (I)) confirmed by 1736 act 9 Geo. 2. c. 9 (I))

Imperial gallon
The British imperial gallon (frequently called simply "gallon") is defined as exactly 4.54609 dm3 (4.54609 litres). It is used in some Commonwealth countries, and until 1976 was defined as the volume of water at 62 F whose mass is 10 lb. There are four imperial quarts in a gallon, two imperial pints in a quart, and there are 20 imperial fluid ounces in an imperial pint, yielding 160 fluid ounces in an imperial gallon.

US liquid gallon
The US liquid gallon (frequently called simply "gallon") is legally defined as 231 cubic inches, which is exactly 231 in3. A US liquid gallon can contain about 3.785 kg of water at 3.98 C, and is about 16.7% less than the imperial gallon. There are four quarts in a gallon, two pints in a quart and 16 US fluid ounces in a US pint, which makes the US fluid ounce equal to $1⁄128$ 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 and alcoholic beverages are both referenced to 60 F in government regulations.

US dry gallon
Since the dry measure is one-eighth of a US Winchester bushel of $2,150.42$ cubic inches, it is equal to exactly 268.8025 cubic inches, which is $4.405 L$. The US dry gallon is not used in commerce, and is also not listed in the relevant statute, which jumps from the dry pint to the bushel.

Imperial gallon
As of 2021, the imperial gallon continues to be used as the standard petrol unit on 10 Caribbean island groups, consisting of:


 * four British Overseas Territories (Anguilla, the British Virgin Islands, the Cayman Islands, and Montserrat)  and
 * six countries (Antigua and Barbuda, Dominica, Grenada, Saint Christopher and Nevis, Saint Lucia, and Saint Vincent and the Grenadines ).

All 12 of the Caribbean islands use miles per hour for speed limits signage, and 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. In 2014, Myanmar switched from the imperial gallon to the litre.

Antigua and Barbuda has proposed switching to selling petrol by litres since 2015.

In the European Union the gallon was removed from the list of legally defined primary units of measure catalogue in the 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.

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. 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.

US liquid gallon
Other than the United States, petrol is sold by the US gallon in 13 other countries, and one US territory:
 * the Caribbean countries of Dominican Republic and Haiti,
 * the Central American countries of Belize, Guatemala, Honduras, and Nicaragua,
 * the South American countries of Colombia, Ecuador, and Peru,
 * the Pacific Ocean countries of Marshall Islands, Federated States of Micronesia, and Palau, which are associated countries of the United States,
 * the African country of Liberia, a former protectorate of the United States, and
 * the US territories of American Samoa, the Northern Mariana Islands, Guam, and the US Virgin Islands. Puerto Rico ceased selling petrol by the US gallon in 1980.

The latest country to cease using the gallon is El Salvador in June 2021.

The Imperial and US liquid gallon
Both the US gallon and imperial 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)) and the Bahamas.

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.

Relationship to other units
Both the US liquid and imperial gallon are divided into four quarts (quarter gallons), which in turn are divided into two pints, which in turn are divided into two cups (not in customary use outside the US), which in turn are further divided into two gills. Thus, both gallons are equal to four quarts, eight pints, sixteen cups, or thirty-two gills.

The imperial gill is further divided into five fluid ounces, whereas the US gill is divided into four fluid ounces, meaning an imperial fluid ounce is $1⁄20$ of an imperial pint, or $1⁄160$ of an imperial gallon, while a US fluid ounce is $1⁄16$ of a US pint, or $1⁄128$ of a US gallon. Thus, the imperial gallon, quart, pint, cup and gill are approximately 20% larger than their US counterparts, meaning these are not interchangeable, but the imperial fluid ounce is only approximately 4% smaller than the US fluid ounce, meaning these are often used interchangeably.

Historically, a common bottle size for liquor in the US was the "fifth", i.e. one-fifth of a US gallon (or 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".

History
The term derives most immediately from galun, galon in Old Norman French, but the usage was common in several languages, for example jale in Old French and gęllet (bowl) in Old English. This suggests a common origin in Romance Latin, but the ultimate source of the word is unknown.

The gallon originated as the base of systems for measuring wine and 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 either the ale gallon or the larger 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 268.8 cubic inches (≈ 4.405 L),
 * The wine gallon, or Queen Anne's gallon, which was 231 cubic inches (≈ 3.785 L), and
 * The ale gallon of 282 cubic inches (≈ 4.622 L).

The corn or dry gallon is used (along with the dry quart and pint) in the United States for grain and other dry commodities. It is one-eighth of the (Winchester) bushel, originally defined as a cylindrical measure of $18 1⁄2$ inches in diameter and 8 inches in depth, which made the bushel 8 in × ($9 1⁄4$ in)2 × $\pi$ ≈ 2150.42017 cubic inches. The bushel was later defined to be 2150.42 cubic inches exactly, thus making its gallon exactly $268.803 in3$ ($4.405 L$); in previous centuries, there had been a corn gallon of between 271 and 272 cubic inches.

The wine, fluid, or liquid gallon has been the standard US gallon since the early 19th century. The wine gallon, which some sources relate to the volume occupied by eight medieval merchant pounds of wine, 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 ≈ 230.907 06 cubic inches. It was redefined during the reign of Queen Anne in 1706 as 231 cubic inches exactly, the earlier definition with π approximated to $22⁄7$.

$$ \pi r^2h \approx \frac{22}{7}\times\left ( \frac{7 ~ \mathrm{in}}{2} \right )^2\times6 ~ \mathrm{in} = 231 ~ \mathrm{in}^3.$$

Although the wine gallon had been used for centuries for import duty purposes, there was no legal standard of it in the Exchequer, while a smaller gallon (224 cu in) was actually in use, requiring this statute; the 231 cubic inch gallon remains the US definition today.

In 1824, Britain adopted a close approximation to the ale gallon known as the imperial gallon, and abolished all other gallons in favour of it. Inspired by the kilogram-litre relationship, the imperial gallon was based on the volume of 10 pounds of distilled water weighed in air with brass weights with the barometer standing at 30 inches of mercury (14.7345 pound-force per square inch) and at a temperature of 62 F.

In 1963, this definition was refined as the space occupied by 10 pounds of distilled water of density $0.999 g/mL$ weighed in air of density $0.001 g/mL$ against weights of density 8.136 g/mL (the original "brass" was refined as the densities of brass alloys vary depending on metallurgical composition), which was calculated as $4.546 L$ to ten significant figures.

The precise definition of exactly $4.546$ cubic decimetres (also $4.546 L$, ≈ $277.419 in3$) came after the litre was redefined in 1964. This was adopted shortly afterwards in Canada, and adopted in 1976 in the United Kingdom.

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.