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 4.54609 litres, which is or was used in the United Kingdom, Ireland, Canada, Australia, New Zealand, and some Caribbean countries;
• the US liquid gallon (US gal), defined as 231 cubic inches (exactly 3.785411784 L),^[1] which is used in the United States and some Latin American and Caribbean countries; and
• the US dry gallon, defined as 1⁄8 US bushel (exactly 4.40488377086 L).

Quick Facts General information, Unit of ...

gallon
A one-US-gallon gas can showing "U.S. Gallon" marking (for American use), imperial gallons (for British use), and litres (for Canadian use)
General information
Unit of	Volume
Symbol	gal
Conversions (imperial)
1 imp gal in ...	... is equal to ...
SI units	4.54609 L
US customary units	≈ 1.200950 US gal
US customary units	≈ 277.4194 in3
Conversions (US)
1 US gal in ...	... is equal to ...
SI units	3.785411784 L
Imperial units	≈ 0.8326742 imp gal
Imperial units	231 in3
US dry gallon	≈ 0.859367 US dry gal

Close

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,^[2] not to be confused with the gal (symbol: Gal), a CGS unit of acceleration.

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.

Imperial gallon

The British imperial gallon (frequently called simply "gallon") is defined as exactly 4.54609 dm³ (4.54609 litres).^[4] It is used in some Commonwealth countries, and until 1976 was defined as the volume of water at 62 °F (16.67 °C)^[5][6] whose mass is 10 pounds (4.5359237 kg). 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,^[4] yielding 160 fluid ounces in an imperial gallon.

US liquid gallon

See also: Fluid volume

The US liquid gallon (frequently called simply "gallon") is legally defined as 231 cubic inches, which is exactly 3.785411784 litres.^[7][8] A US liquid gallon can contain about 3.785 kilograms or 8.34 pounds of water at 3.98 °C (39.16 °F), 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^[9] and alcoholic beverages^[10] are both referenced to 60 °F (15.6 °C) in government regulations.

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".^[50][51]

The term derives most immediately from galun, galon in Old Norman French,^[52] 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.^[53]

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^[54] (≈ 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 × π ≈ 2150.42017 cubic inches. The bushel was later defined to be 2150.42 cubic inches exactly, thus making its gallon exactly 268.8025 in³ (4.40488377086 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^{[citation needed]}. 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.

$${\displaystyle \pi r^{2}h\approx {\frac {22}{7}}\times \left({\frac {7~\mathrm {in} }{2}}\right)^{2}\times 6~\mathrm {in} =231~\mathrm {in} ^{3}.}$$

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^{[citation needed]}, 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 (100 kilopascals; 15 pounds per square inch) and at a temperature of 62 °F (17 °C).

In 1963, this definition was refined as the space occupied by 10 pounds of distilled water of density 0.998859 g/mL weighed in air of density 0.001217 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 4.546091879 L to ten significant figures.^[5]

The precise definition of exactly 4.54609 cubic decimetres (also 4.54609 L, ≈ 277.419433 in³) came after the litre was redefined in 1964. This was adopted shortly afterwards in Canada, and adopted in 1976 in the United Kingdom.^[5]