Two gallons, one name
The word 'gallon' is used in both the United States and the United Kingdom, but the two countries do not mean the same volume by it. A US (liquid) gallon is defined as exactly 3.785411784 liters, while a UK imperial gallon is defined separately, as exactly 4.54609 liters -- about 20% larger than the US gallon. Because both units share the same everyday name, this size difference is a frequent, easy-to-miss source of error when comparing recipes, fuel prices, container sizes or fuel-economy figures across US and UK or Commonwealth sources.
The history: an old wine gallon and a 19th-century reform
The US gallon descends from an English wine gallon standard, historically fixed at exactly 231 cubic inches, commonly dated to an English statute of the early 18th century. The American colonies, and later the independent United States, retained this wine-gallon-based definition even as the volume was progressively re-expressed in metric terms; 231 cubic inches, converted using the exact inch-to-metric relationship, equals exactly 3.785411784 liters.
The United Kingdom took a different path in the 19th century: the Weights and Measures Act 1824 established a single imperial gallon for the UK, replacing the assortment of older wine, ale and corn gallons that had been in use, and originally defined it as the volume occupied by 10 pounds (avoirdupois) of distilled water weighed in air against brass weights at a specified temperature. The modern legal definition, fixed in metric terms under the UK's Weights and Measures Act 1985, sets the imperial gallon at exactly 4.54609 liters. Because the US kept its older wine-gallon-derived standard while the UK adopted a newly defined, larger imperial gallon, the two countries ended up with two different volumes sharing the same everyday name.
The exact modern values
Both gallon definitions used today are exact, not rounded approximations of some other quantity: 1 US gallon = 3.785411784 liters exactly, and 1 UK (imperial) gallon = 4.54609 liters exactly. Dividing the two gives the exact size ratio between them: a UK gallon holds about 1.201 times as much as a US gallon, or roughly 20% more.
This means the same physical container described as 'one gallon' will hold a measurably different amount of liquid depending on which gallon definition applies. Ten US gallons is exactly 37.85411784 liters, while ten UK gallons is exactly 45.4609 liters -- a difference of about 7.6 liters for what both sources would label simply as '10 gallons.'
| Quantity | US gallons | UK (imperial) gallons |
|---|---|---|
| 1 gallon | 3.785411784 L | 4.54609 L |
| 10 gallons | 37.85411784 L | 45.4609 L |
| Size ratio (UK ÷ US) | 1.000 | ≈1.201 |
MPG consequences
Because miles per gallon (MPG) is distance traveled divided by fuel volume consumed, and the two countries' gallons are different sizes, the same real-world fuel economy produces two different MPG numbers depending on which gallon definition is used -- not because the car performs differently, but because the unit of fuel volume in the denominator is different. Since the UK imperial gallon holds about 20% more fuel than the US gallon, a UK gallon carries a vehicle proportionally further, producing a numerically higher miles-per-gallon figure for exactly the same underlying fuel consumption.
This is why a vehicle's fuel economy quoted in a British or Commonwealth publication will typically look better, as a raw number, than the same vehicle's fuel economy quoted in an American publication, even when both describe identical real-world performance. Converting correctly between MPG (US), MPG (UK) and the metric measure liters per 100 km requires knowing which gallon definition is in play; a closer look at the reciprocal relationship between MPG and L/100km is covered in this site's guide on comparing fuel economy units.
Other US/imperial divergences: pints and fluid ounces
The gallon is not the only volume unit where the US and imperial systems part ways -- because both the pint and fluid ounce are defined as fractions of their respective gallon, the same size difference cascades down through the smaller units. A US pint is 1/8 of a US gallon (16 US fluid ounces), equal to exactly 473.176473 milliliters, while a UK (imperial) pint is 1/8 of an imperial gallon (20 imperial fluid ounces), equal to exactly 568.26125 milliliters -- notably, the US pint has 16 fluid ounces while the UK pint has 20, so the ounce count itself differs, on top of the underlying gallon size difference.
The fluid ounce diverges too: a US fluid ounce is exactly 29.5735295625 milliliters, while a UK (imperial) fluid ounce is exactly 28.4130625 milliliters -- the US fluid ounce is actually slightly larger than the UK one, even though the US pint and gallon are both smaller than their UK counterparts, because the two systems divide their respective gallons into a different number of ounces. This combination of differences is why converting any US customary volume to its UK imperial equivalent requires the actual defined constants for the specific unit involved, rather than assuming a single conversion factor applies across pints, fluid ounces and gallons alike.
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Is a US gallon the same size as a UK gallon?
No. A US (liquid) gallon is exactly 3.785411784 liters, while a UK (imperial) gallon is exactly 4.54609 liters -- about 20% larger. The two were defined independently by separate historical standards, so a US gallon figure cannot be read as a UK gallon figure, or vice versa, without converting.
Why did the US and UK end up with different gallons?
The US gallon descends from an older English wine gallon standard, fixed at exactly 231 cubic inches, which the US retained. The UK instead reformed its measures under the Weights and Measures Act 1824, establishing a single, larger imperial gallon to replace several older wine, ale and corn gallons, later fixed in metric terms at exactly 4.54609 liters under the Weights and Measures Act 1985.
Why is UK MPG always a higher number than US MPG for the same car?
Because the UK imperial gallon holds about 20% more fuel than the US gallon, so a UK gallon carries a vehicle proportionally further for the same real-world fuel efficiency, producing a numerically higher miles-per-gallon figure. The car's actual performance has not changed -- only the size of the gallon used in the denominator of the MPG calculation has.
How many liters are in 10 US gallons versus 10 UK gallons?
10 US gallons is exactly 37.85411784 liters, while 10 UK (imperial) gallons is exactly 45.4609 liters -- a difference of about 7.6 liters for a quantity both sources would casually describe as '10 gallons.'
Do the US and UK also differ on pints and fluid ounces?
Yes. A US pint is 16 US fluid ounces (473.176473 mL), while a UK pint is 20 imperial fluid ounces (568.26125 mL) -- a different ounce count as well as a different total size. The fluid ounce itself also differs: the US fluid ounce (29.5735295625 mL) is actually slightly larger than the UK fluid ounce (28.4130625 mL), even though the US gallon and pint are both smaller overall.
Kaynaklar
- National Institute of Standards and Technology (NIST) -- Special Publication 811, exact US customary volume unit definitions (gallon, pint, fluid ounce).
- United Kingdom. Weights and Measures Act 1985, Schedule 1 -- definition of the imperial gallon as exactly 4.54609 liters.
- United Kingdom. Weights and Measures Act 1824 -- historical establishment of a single UK imperial gallon, replacing earlier wine, ale and corn gallons.
- NIST Handbook 44, Appendix C -- General Tables of Units of Measurement (US gallon, pint and fluid ounce definitions).