This converts a volume value you already know — distinct from the volume calculator, which works one out from a solid’s dimensions. It also keeps the two gallons — US and imperial — clearly separate, because treating them as the same unit is one of the most common conversion mistakes there is.
How it works
Exact definitions
1 US gallon = 231 in³ = 3.785411784 L exact
1 imperial gallon = 4.54609 L exact
The imperial gallon is about 20% larger than the US gallon — both are shown rather than one being silently assumed, the same rule the volume calculator follows for its own unit choices.
Why the two gallons matter
A US gallon of milk and a UK gallon of petrol are not the same amount, and the gap is large enough to matter — roughly 20%. This trips people up most often with fuel economy figures: “40 miles per gallon” means a genuinely different real-world efficiency depending on which gallon is meant, which is exactly why this site’s fuel economy converter treats US and UK mpg as separate figures rather than one number.
How to use this calculator
- Type a volume into any field.
- The rest update — litres, millilitres and both gallons up front; cubic metres, cups, tablespoons and cubic feet behind “More units”.
Frequently asked questions
Why are there two different gallons?
Historical divergence — the US kept an older definition after the UK standardised a different one in 1824. Both are still in everyday use in their respective countries, which is why recipes, fuel figures and container sizes need to specify which gallon they mean.
Which gallon is bigger?
The imperial (UK) gallon, by about 20% — 4.54609 litres against 3.785411784 for the US gallon.
Is a US cup a fixed, exact volume?
The US legal cup used here is exactly 1/16 of a US gallon. Some recipe sources use a slightly different “customary” cup, close enough that the difference rarely matters in home cooking.
Are these conversions exact?
Yes — both gallon definitions and the metric relationships are exact by definition, not rounded approximations.
