The US gallon and the UK (Imperial) gallon are different because the two countries adopted their standards at different points in history, using completely different methods to define the volume. The US gallon is 231 cubic inches (3.785 liters), while the Imperial gallon is 277.42 cubic inches (4.546 liters), making the UK gallon about 20% larger. The split happened because the United States kept an older English measure while Britain replaced it with a new one in 1824.
The US Kept an Older English Standard
Before American independence, the colonies used the same measurement systems as England, including a unit called the wine gallon. This gallon had been formally defined by a 1706 statute under Queen Anne as exactly 231 cubic inches, roughly the volume of a cylinder 7 inches across and 6 inches tall. It was the standard for measuring wine, cooking oils, honey, and other commercial liquids.
After independence, the young United States simply continued using the measures it already had. The wine gallon remained the basis for liquid measurement, and in 1836 the US Treasury formally adopted it as the country’s official gallon. There was no pressing reason to change a system that merchants and farmers already understood.
Britain Redefined Its Gallon in 1824
England, meanwhile, had accumulated a tangle of competing gallons over the centuries. There was the wine gallon for liquids, the corn (Winchester) gallon for dry goods, and the ale gallon for beer. Parliament decided to simplify things with the Weights and Measures Act of 1824, which scrapped all previous gallons and replaced them with a single new Imperial gallon.
Rather than basing the new gallon on a geometric shape, lawmakers defined it by the weight of water: one Imperial gallon equaled the volume occupied by exactly 10 pounds of distilled water, weighed at 62 °F with barometric pressure at 30 inches. That came out to 277.42 cubic inches, noticeably larger than the old 231-cubic-inch wine gallon. Britain and its colonies switched to this new Imperial system in 1826. The United States, already independent for half a century, had no reason to follow.
How the Two Gallons Compare in Practice
The size gap between the two gallons cascades through every related unit, but not in a straightforward way. Each system divides the gallon differently, so the sub-units don’t scale evenly.
- Gallon: 1 US gallon = 3.785 liters. 1 Imperial gallon = 4.546 liters. The Imperial gallon is about 20% larger.
- Pint: A US pint contains 16 fluid ounces, while an Imperial pint contains 20. That makes the UK pint significantly bigger, which is why a pint of beer in a London pub looks generous compared to one in New York.
- Fluid ounce: Here the relationship flips slightly. A US fluid ounce is 29.57 mL, while an Imperial fluid ounce is 28.41 mL. The US ounce is actually a bit larger, but because there are fewer of them in a US pint, the US pint still ends up smaller overall.
Why Fuel Economy Numbers Look Different
One place this causes real confusion is fuel economy. A car rated at 30 miles per gallon in the US would be rated at about 36 miles per gallon using Imperial gallons, because the Imperial gallon is larger and therefore takes the car farther. The conversion factor is roughly 1.201: multiply US MPG by 1.201 to get the Imperial equivalent. If you’re comparing car specs between American and British sources without accounting for this, the UK numbers will always look better by about 20%.
The Forgotten Dry Gallon
Adding one more wrinkle, the US also once had a dry gallon used for grain and other non-liquid goods. This one descended from the Winchester bushel, a standard dating back to Henry VII in 1495. The dry gallon was defined as one-eighth of that bushel, working out to about 268.8 cubic inches (4.405 liters), placing it between the US liquid gallon and the Imperial gallon in size. It fell out of practical use by the 1990s and is no longer included in US statutes, though the Winchester bushel itself remains the legal standard for grain trading.
Why the Difference Persists
Most countries have since moved to the metric system, making the whole question irrelevant for international trade. The UK officially adopted metric units for most purposes in the late 20th century, though Imperial measures survive in everyday life for things like pints of milk and miles on road signs. The US remains one of the few countries where customary units dominate daily commerce. Since neither country has fully abandoned its traditional system, the two different gallons continue to coexist, a lasting artifact of a decision Britain made in 1824 and the United States chose not to follow.