Two different gallons
- US gallon: 3.785 liters, used in the United States.
- UK (imperial) gallon: 4.546 liters, about 20% bigger, still used for fuel economy in the UK.
The US gallon comes from an old English wine gallon; Britain redefined its gallon in 1824 as the volume of 10 pounds of water. The two have stayed apart ever since.
The smaller units differ too
- Pint: US 473 ml, UK 568 ml. A UK pint of beer is noticeably bigger.
- Quart: US 946 ml, UK 1,137 ml.
- Fluid ounce: US 29.57 ml, UK 28.41 ml. Here the US unit is the larger one.
A US gallon has 128 US fluid ounces; a UK gallon has 160 UK fluid ounces.
Miles per gallon
Because the UK gallon is bigger, the same car gets a higher mpg figure in the UK. Multiply UK mpg by 0.833 to get US mpg: 50 mpg in a British review is about 42 mpg in American terms.
Converting
The Gallons to Liters and Liters to Gallons converters use US gallons. The Volume Converter has both US and UK gallons and pints, so you can convert between them directly.