There are two gallons, and they are not close
Most unit confusions involve a few percent. This one involves a fifth.
A US gallon is 3.785411784 liters. An imperial gallon, still used in the United Kingdom and a handful of other places, is 4.54609 liters. The imperial gallon is 20.09 percent larger, which is enough to matter for almost anything you would measure in gallons.
So this converter shows both, every time. Guessing which one a source meant is exactly the mistake worth designing out.
The formula
Imperial gallons = liters ÷ 4.54609
Both figures are exact. The US gallon is defined as 231 cubic inches, and since an inch is exactly 2.54 centimetres, the liter figure follows without any measurement. The imperial gallon was fixed at exactly 4.54609 liters in 1985.
Worked example
A 50 liter fuel tank.
US: 50 ÷ 3.785411784 = 13.21 US gallons
Imperial: 50 ÷ 4.54609 = 11.00 imperial gallons
The same tank, and the two answers are more than two gallons apart. If you were comparing a quoted fuel consumption figure without checking which gallon it used, that gap would land straight in your arithmetic.
Why British cars look more efficient than they are
This is the most common place the two gallons collide in ordinary life.
Miles per gallon means US gallons in America and imperial gallons in Britain. Since the imperial gallon is a fifth larger, a car goes a fifth further on one, so the same car scores about 20 percent higher in British figures than American ones.
| Real consumption | Quoted in the US | Quoted in the UK |
|---|---|---|
| Identical car | 30 mpg | 36 mpg |
| Identical car | 40 mpg | 48 mpg |
| Identical car | 50 mpg | 60 mpg |
Nobody is being dishonest here. Both numbers are correct in their own system, and both countries have used their own gallon for two centuries. It is simply the same car divided by two different things, which is worth remembering before concluding that European cars are dramatically thriftier than American ones. Some of that difference is real. About twenty points of it is the gallon.
Where the two gallons came from
The United States inherited the English wine gallon of 231 cubic inches and kept it after independence. Britain then reformed its own measures in 1824, defining the gallon as the volume occupied by ten pounds of water at a specified temperature, which came out noticeably larger.
Neither side had much reason to change afterwards, so both survived, and here we are. There is also a US dry gallon of about 4.405 liters used for grain, which you will almost certainly never meet.
For smaller volumes the ml to oz converter covers the same ground at kitchen scale, and if you are filling something rather than converting a number, the pool volume calculator works out how much goes in.