Three gallons, one word
"Gallon" names three unrelated volumes: the US liquid gallon, the much rarer US dry gallon, and the Imperial gallon used in the UK and much of the Commonwealth before metrication. None is a rounding of another. Each was fixed by a different legal act, in a different country, for a different trade, and the largest of the three is about 20% bigger than the smallest.
US liquid gallon: a pre-1824 wine gallon that never left
The US liquid gallon is defined as exactly 231 cubic inches, which converts to exactly 3.785411784 L (231 × 16.387064 cm³, since the inch has been exactly 2.54 cm since 1959). That 231 in³ figure is not an American invention: it is the old English "wine gallon," standardized under Queen Anne in 1707 for measuring wine and spirits, and it is what British colonists in North America used and kept using. In 1824, Britain replaced its wine gallon, ale gallon and corn gallon with a single Imperial gallon defined as the volume of 10 lb of distilled water weighed in air at 62°F — about 277.42 in³, noticeably bigger than the old wine gallon. The newly independent United States had no reason to follow a British law passed decades after independence, and never adopted it. The US gallon on every gas pump and milk carton today is, in effect, a fossil of pre-1824 English measurement, preserved by the accident of timing.
US dry gallon: the one almost nobody encounters
A second, larger US gallon exists for dry goods: fruit, grain and produce sold by volume. It derives from the Winchester bushel, fixed by US statute at exactly 2150.42 in³. A dry gallon is 1/8 of that bushel: 2150.42 ÷ 8 = 268.8025 in³, which converts to 4.404883770… L — about 16.4% larger than the liquid gallon despite sharing its name. In practice the dry gallon itself is rarely quoted; its subdivisions (the dry quart, the peck, the bushel) survive in US agricultural and horticultural trade, while the dry gallon sits between them mostly unused.
Imperial gallon: redefined once in 1824, fixed exactly in 1985
Britain's 1824 Weights and Measures Act defined its new gallon physically, by weighing water, which made it dependent on the precision of that weighing and on stated conditions of temperature and pressure. That physical definition was retired by the UK's Weights and Measures Act 1985, which fixed the Imperial gallon at exactly 4.54609 L — a defined constant, no longer tied to weighing anything. The Imperial gallon has been legally obsolete for retail trade in the UK since metrication, but it survives in fuel-economy figures on older vehicles, in some drinks measures, and across parts of the Commonwealth.
The three gallons side by side
| Gallon | Exact value | Defined as | Ratio to US liquid gallon |
|---|---|---|---|
| US liquid | 3.785411784 L | 231 in³ exactly | 1.000 |
| US dry | 4.404883770… L | 1/8 US bushel (2150.42 in³) | 1.164 |
| Imperial | 4.54609 L exactly | Fixed by UK statute, 1985 | 1.201 |
Why this matters more than it looks: the fluid ounce count itself changes
The three-ounces guide on this site already covers the exact sizes of the US fluid ounce (29.5735295625 mL) and the Imperial fluid ounce (28.4130625 mL) in detail, so those derivations aren't repeated here. What's worth adding is how the two gallons carve themselves up differently: a US gallon is defined as 128 US fluid ounces (4 quarts × 2 pints × 2 cups × 8 fl oz), while an Imperial gallon is defined as 160 Imperial fluid ounces (4 quarts × 2 pints × 20 fl oz, with no "cup" step at all in the traditional Imperial system). Both facts are true at once: the Imperial gallon holds more liquid than the US gallon, and it is also divided into more, smaller-numbered fluid ounces. A US gallon container and an Imperial gallon container are different sizes for two independent reasons stacked on top of each other — the base unit differs, and the subdivision count differs too.
The pint inherits the same split
The pattern that produces two different fluid ounces and two different gallons runs straight through every subdivision in between. A US pint is 1/8 of a US gallon, or 16 US fluid ounces: 16 × 29.5735295625 mL = 473.176473 mL. An Imperial pint is 1/8 of an Imperial gallon, or 20 Imperial fluid ounces (Imperial measures never used a "cup" step at all): 20 × 28.4130625 mL = 568.26125 mL. The ratio between them, 568.26125 ÷ 473.176473 = 1.20095, is exactly the same 20.1% gap as between the two gallons, because a pint is just a gallon divided by the same constant, 8, in both systems. "A pint's a pound the world around" was never quite true even within English-speaking countries: an Imperial pint of water weighs close to 20 oz, a US pint close to 16.7 oz, because the volumes themselves differ by a fifth.
Where it bites
US fuel-economy figures (miles per US gallon) and older UK or Commonwealth figures (miles per Imperial gallon) are not comparable without converting the gallon first, a point the mpg guide on this site works through with numbers. Recipes, paint coverage estimates and any US/UK cooking or DIY conversion that assumes "a gallon is a gallon" is quietly off by 20% the moment an Imperial source meets a US-labeled container, and by a further few percent again if a US dry-goods gallon gets mixed in by mistake.