Gallon

• 1. Introduction

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  The gallon is one word with two different sizes, and the gap between them is large enough to disguise a tax increase. In the Turks and Caicos Islands, authorities once levied the same duty on the US gallon as had previously been charged on the larger imperial gallon. One measure holds 4.54609 litres. The other is fixed at exactly 231 cubic inches. Both answer to the same name and the same symbol, gal. A driver in Britain and a driver in the United States can each fill up by the gallon and walk away with noticeably different amounts of fuel. How did a single unit split into two? Why does the United Kingdom still calculate fuel consumption in a measure it no longer allows shops to sell by? And how did medieval merchants' pounds of wine, a cylinder six inches deep, and an Order in Council all shape a number still used at the petrol pump today?

• 2. Two Gallons, One Name

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  The imperial gallon equals exactly 4.54609 litres, while the US gallon is defined as exactly 231 cubic inches. The two are built from the same scaffolding but with one crucial difference deep inside. Both gallons divide into four quarts, eight pints, and thirty-two gills. The split happens at the smallest level. The imperial gill is divided into five fluid ounces, but the US gill is divided into only four. That difference, combined with a slight difference in the sizes of the imperial and US fluid ounces, gives the two gallons their different volumes. A US gallon contains 3.785411784 kilograms of water at 3.983 degrees Celsius. The imperial gallon, until 1976, was defined as the volume of water at 62 degrees Fahrenheit whose mass is 10 pounds. The shared symbol gal can mislead in another way. It looks identical to Gal, the CGS unit of acceleration, though that one carries a capital G.

• 3. A Wine And Beer Inheritance

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  The gallon originated as the basis of systems for measuring wine and beer in England, and the two systems never agreed. The wine gallon was equal in size to today's US gallon. The ale gallon ran 1.65 percent larger than the imperial gallon. By the end of the 18th century, three definitions were in common use at once. The corn gallon, also called the Winchester gallon, held about 268.8025 cubic inches. The wine gallon, also called Queen Anne's gallon, held 231 cubic inches. The ale gallon held 282 cubic inches. The word itself arrived through galun, galon in Old Norman French, with relatives like jale in Old French and gęllet, meaning bowl, in Old English. That spread across languages points to a common origin in Romance Latin, but the ultimate source of the word is unknown. The term for a capacity measured in gallons is gallonage.

• 4. The Numbers Behind The Standards

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  In 1706, during the reign of Queen Anne, the wine gallon was redefined as exactly 231 cubic inches. Some sources tie the wine gallon to the volume occupied by eight medieval merchants' pounds of wine, once defined as a cylinder six inches deep and seven inches in diameter. The wine gallon had been used for centuries for import duty, yet there was no legal standard of it in the Exchequer, and a smaller gallon of 224 cubic inches was actually in use. The wine gallon was legally adopted as the standard US gallon in 1836, and 231 cubic inches remains the definition of a gallon in the United States today. Britain went a different way. In 1824, Britain adopted the imperial gallon and abolished all other gallons in favour of it. It was first defined as the volume of 10 pounds of distilled water weighed in air with brass weights, the barometer at 30 inches of mercury and the temperature at 62 degrees Fahrenheit, calculated as 277.274 cubic inches. An Order in Council of the 28th of November 1889 redefined it as 277.463 cubic inches. A 1963 refinement, accounting for water density, air density, and weights of density 8.136 grams per millilitre, produced about 277.41955 cubic inches. The clean figure of exactly 4.54609 cubic decimetres followed the litre's redefinition in 1964. Canada adopted it shortly after, and the United Kingdom in 1976.

• 5. Where The Gallon Still Pumps

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  Eight Caribbean territories still sell petrol by the imperial gallon, all of which use miles per hour for speed limit signs and drive on the left. They are four British Overseas Territories, namely Anguilla, the British Virgin Islands, the Cayman Islands, and Montserrat, plus four countries, namely Antigua and Barbuda, Grenada, Saint Lucia, and Saint Vincent and the Grenadines. The US gallon sells gasoline across four US territories, including American Samoa, the Northern Mariana Islands, Guam, and the US Virgin Islands, and in six countries, including the United States, Guatemala, Colombia, Ecuador, Peru, Haiti, the Marshall Islands, the Federated States of Micronesia, Palau, and Liberia, a former protectorate of the United States. Some places hedge their bets. Both gallons are used in the Turks and Caicos Islands and in the Bahamas. The retreat has been steady. The United Arab Emirates stopped selling petrol by the imperial gallon in 2010 and switched to the litre, Guyana followed in 2013, and Myanmar switched in 2014. In March 2025, the Dubai Electricity and Water Authority moved water billing from imperial gallons to cubic metres. Antigua and Barbuda has proposed switching to litres since 2015.

• 6. Legal Twilight

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  It is not legal to sell items by the imperial gallon in Britain, yet people there still use it for fuel consumption. That contradiction grew out of European rules. The EU directive 80/181/EEC removed the gallon from the list of legally defined primary units for trade and official purposes, effective from the 31st of December 1994. Under the directive, the gallon could survive only as a supplementary or secondary unit. Ireland and the United Kingdom both passed legislation in response. Ireland replaced the gallon with the litre as a primary trade unit effective from the 31st of December 1993, and the United Kingdom did so effective from the 30th of September 1995. The gallon kept a foothold anyway. It can still be legally used in both the UK and Ireland as a supplementary unit. Miles per imperial gallon remains the primary fuel economy unit in the United Kingdom and a supplementary unit in Canada on official documentation.

• 7. Fifths, Bushels, And Lost Measures

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  A common bottle size for liquor in the United States was once the fifth, meaning one-fifth of a US gallon. That made it 0.08 percent more than a reputed quart, which is one-sixth of an imperial gallon. Spirit sales in the US switched to metric measures in 1976, yet a 750 millilitre bottle is still sometimes called a fifth. Dry goods had their own gallon. The US dry gallon was one-eighth of a US Winchester bushel of exactly 2150.42 cubic inches, which works out to 268.8025 cubic inches. The bushel began as a cylindrical measure later redefined to 2,150.42 cubic inches exactly. In earlier centuries there had been a corn gallon of between 271 and 272 cubic inches. The dry gallon served the United States for grain and other dry commodities until the 1990s. It is no longer used and no longer appears in the relevant statute, which now jumps from the dry quart straight to the peck. Old habits linger in odd places. In parts of the Middle East, including the United Arab Emirates and Bahrain, 18.9-litre water cooler bottles are marketed as five-gallon bottles.

• 8. A Cabinet Of Vanished Gallons

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  The Roman congius held about 216 cubic inches, the smallest entry in a long catalogue of gallons that once filled Western Europe. The Irish gallon of 1495, re-confirmed in 1736, came to about 217 cubic inches. An old UK wine gallon of 224 cubic inches survives, preserved at the Guildhall in London. The Channel Islands kept their own, with a Jersey gallon dated to 1562 and a Guernsey gallon of 17th-century origins that lasted until 1917. The records reach deep into English history, listing an Ancient Rumford quart of 1228, an Exchequer gallon tied to Henry VII in 1497, and an Elizabeth I corn gallon of 1601. The beer and ale gallon used in the Treasury before 1824 stood at 282 cubic inches. Across these localities, the litre has replaced the gallon as the unit of capacity, leaving these figures as a measure of how many different answers one word once carried.