Gallon

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.

The English system gallons

There were more than a few systems of liquid measurements in the pre-1884 United Kingdom.

The Imperial gallon

The imperial (UK) gallon, now defined as exactly 7000454609000000000♠4.54609 litres (about 277.42 cubic inches), is used in some Commonwealth countries and was originally based on the volume of 10 pounds (approximately 4.54 kg) of water at 62 °F (17 °C). The imperial fluid ounce is defined as  ¹⁄₁₆₀ of an imperial gallon; there are four quarts in a gallon, two pints in a quart, and 20 Imperial fluid ounces in an imperial pint.

The US liquid gallon

The US gallon is legally defined as 231 cubic inches, which is exactly 7000378541178400000♠3.785411784 liters. A US liquid gallon of water weighs about 8.34 pounds or 3.78 kilograms at 62 °F (17 °C), making it about 16.6% lighter 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 (16 °C) in government regulations.

The US dry gallon

This dry measure is one-eighth of a US Winchester bushel of 7003215042000000000♠2150.42 cubic inches; it is therefore equal to exactly 268.8025 cubic inches or about 6997440500000000000♠4.405 L. The US dry gallon is not used in commerce, and is not listed in the relevant statute, which jumps from the dry quart to the peck.

Worldwide usage of gallons

The Imperial gallon is used in everyday life (and in advertising) in the United Kingdom, and less frequently in Canada, including fuel economy expression in advertisements and other official publications. Gallons used in fuel economy expression in Canada are Imperial gallons.

The gallon was removed from the list of legally defined primary units of measure catalogued in the EU directive 80/181/EEC, for trading and official purposes, with effect from 31 December 1994. Under the directive the gallon could still be used – but only as a supplementary or secondary unit. One of the impacts of this directive was that the United Kingdom amended its own 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 30 September 1995.

Ireland also passed legislation in response to the EU directive with the effective date being 31 December 1993. Though the gallon has ceased to be the legally defined primary unit, it can still be legally used in both the UK and Ireland as a supplementary unit.

The Imperial gallon continues to be used as a unit of measure in Anguilla, Antigua and Barbuda, the Bahamas, the British Virgin Islands, the Cayman Is., Dominica, Grenada, Montserrat, Myanmar (Burma), St. Kitts & Nevis, St. Lucia, and St. Vincent & the Grenadines.

Other than the United States, the US gallon is used in Liberia, Belize, Colombia, The Dominican Republic, Ecuador, El Salvador, Guatemala, Haiti, Honduras, Nicaragua, and Peru.

The United Arab Emirates started selling gasoline by the litre in 2010, along with Guyana, and Panama in 2013. The two former had used the Imperial gallon, and the latter the US gallon until they switched.

Antigua and Barbuda plan to switch over to using litres by 2015.

Despite its status as a U.S. territory, and unlike American Samoa, the Northern Mariana Islands, Guam, and the U.S. Virgin Islands, Puerto Rico ceased selling gasoline by the US gallon in 1980.

In the Turks & Caicos Islands, both the U.S. gallon and Imperial gallon are used, due to an increase in tax duties disguised by levying the same duty on the 3.79 L U.S. gallon as was previously levied on the 4.55 L Imperial gallon.

Ten-gallon hat

Some cowboy hats have been called "ten-gallon" hats. The term came into use about 1925. The Stetson hat company boasted that the tight weave of most Stetson hats made them sufficiently waterproof to be used as a bucket. Early print advertising by Stetson showed a cowboy giving his horse a drink of water from a hat. However, even the Stetson company notes that a "ten-gallon" hat holds only 3 quarts (about 3 L instead of about 38 L).

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. These pints are divided into two cups (though the imperial cup is rarely used now), which in turn are divided into two gills (gills are also rarely used). Thus a gallon is 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. Thus 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.

The imperial gallon, quart, pint, cup and gill are approximately 20% larger than their US counterparts and are therefore not interchangeable. The imperial fluid ounce, on the other hand, is only 4% smaller than the US fluid ounce and therefore they are often used interchangeably.

History

The term derives most immediately from galun, galon in Old Northern 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 on either the ale gallon or the larger imperial gallon.

By the end of the 18th century, three definitions of the gallon were in common use:

The corn or dry gallon was used in the United States until recently for grain and other dry commodities. It is one-eighth of the (Winchester) bushel, originally a cylindrical measure of  18 ¹⁄₂ inches in diameter and 8 inches in depth. That made the dry gallon ( 9 ¹⁄₄)² × π cubic inches ≈ 6997440488409860128♠268.80252 in³. The bushel, which like dry quart and pint still sees some use, was later defined to be 2150.42 cubic inches exactly, making its gallon exactly 6997440488377086000♠268.8025 in³ (6997440488377086000♠4.40488377086 L). In previous centuries, there had been a corn gallon of around 271 to 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 had been redefined during the reign of Queen Anne, in 1706, as 231 cubic inches exactly, which is the result of the earlier definition with π approximated to  ²²⁄₇. 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 (224 cu in) was actually in use, so this statute became necessary. It 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 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 7002998859000000000♠0.998859 g/mL weighed in air of density 7000121700000000000♠0.001217 g/mL against weights of density 8.136 g/mL (the original "brass" was refined as the density of brass alloys vary depending on metallurgical composition). This works out at approximately 6997454609030000000♠4.5460903 L (6997454609028199480♠277.41945 in³). The metric definition of exactly 7000454609000000000♠4.54609 cubic decimetres (also 6997454609000000000♠4.54609 L after the litre was redefined in 1964, ≈ 6997454609000341471♠277.419433 in³) was adopted shortly afterwards in Canada, but from 1976 the conventional value of 6997454609200000000♠4.546092 L was used in the United Kingdom until the Canadian convention was adopted in 1985.

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.