US gallons, imperial gallons and liters compared
The three gallons in common use, why the US liquid gallon is exactly 3.785411784 liters, and how to pick the right one.
Three gallons, three sizes
The word gallon points at more than one volume depending on where and what you are measuring. The US liquid gallon is 3.785411784 liters and covers fuel, milk and most drinks in the United States. The US dry gallon is larger at about 4.404884 liters and is used for dry goods like berries and grain. The imperial gallon of the UK and several Commonwealth countries is larger still at 4.54609 liters. A number quoted in gallons is ambiguous until you know which of these it means.
Where the US figure comes from
The US liquid gallon traces back to the old English wine gallon, fixed at 231 cubic inches. Convert 231 cubic inches into metric and you get exactly 3.785411784 liters, which is why the factor has so many decimal places yet is still called exact. Because the definition is legally fixed rather than measured, the conversion never drifts, and any rounding you see is only for readability.
Why the choice of gallon matters
The gap between the units is big enough to change real decisions. A car rated at 40 miles per US gallon covers the same distance on less fuel than one rated at 40 miles per imperial gallon, because the imperial gallon holds about 20 percent more. A recipe or a tank spec written in imperial gallons will come out short if you treat it as US gallons. When a source does not say, the country of origin is usually the best clue: US figures are almost always US liquid gallons.
A quick liters reference
For fast mental math, one liter is close to a quarter of a US gallon, so four liters is a little over one gallon and ten liters is about two and two thirds gallons. Scaling up, 20 liters is about 5.28 gallons and 100 liters is about 26.42 gallons. These round numbers are handy at the pump or the store, while the box above gives the precise value whenever you need it.