US gallon vs imperial gallon: why the numbers differ
The US and UK gallon are not the same size. Here is where each definition came from and how to avoid mixing them up in a conversion.
Two gallons, one name
The word gallon hides two different volumes that survive in everyday use. The US liquid gallon is defined as exactly 3.785411784 liters, while the imperial gallon used in the United Kingdom is about 4.546 liters. That is a difference of roughly 20 percent, large enough to matter for fuel economy figures, recipe volumes, and tank capacities. This converter works in US gallons throughout, so a UK source needs the imperial value instead.
Where the definitions came from
The US gallon traces back to the old English wine gallon of 231 cubic inches, which the young United States kept even as Britain moved on. In 1824 the UK redefined its gallon as the volume of ten pounds of water, landing on the larger imperial gallon still used today. Because the two systems diverged rather than agreed, the same word now points at two fixed but unequal amounts. Knowing which lineage a figure comes from is the key to converting it correctly.
How the exact factor is built
The US gallon is pinned to the inch, which is defined as exactly 0.0254 meters. Working through 231 cubic inches gives precisely 3.785411784 liters, with no rounding hidden in the definition. That is why this tool can call the conversion exact and only round the displayed digits. When you enter 15 gallons and see 56.781177 liters, the underlying product is exact and just trimmed to six decimal places for a tidy box.
Avoiding the mix-up in practice
Miles per gallon is the classic trap, since a car rated at 40 US mpg is not 40 imperial mpg; the imperial figure is higher because the imperial gallon is bigger. The same care applies to fuel prices per gallon quoted on either side of the Atlantic. When in doubt, convert everything to liters first, which are unambiguous, then compare. This converter gives you the liters figure directly, so you can sidestep the gallon-versus-gallon confusion entirely.