The gallon problem
Volume is the dimension where the same English word means two different amounts depending on which side of the Atlantic wrote it down. A US liquid gallon is exactly 231 cubic inches, which works out at 3.785 411 784 litres. An imperial gallon is exactly 4.546 09 litres. That is a gap of about 20%, under one word, with no marker in the text to tell you which was meant.
The quart and the pint inherit the same gap, because both are simple fractions of their own gallon. A US pint is 473 mL and an imperial pint is 568 mL — which is why a “pint” of beer is noticeably larger in London than in New York, and why a recipe that calls for a pint of stock is ambiguous unless it says whose.
Then the fluid ounce reverses it. A US fluid ounce is 29.57 mL and an imperial fluid ounce is 28.41 mL — the US one is about 4% larger, the opposite direction to the gallon. The reason is that the imperial gallon divides into 160 fluid ounces while the US gallon divides into 128. So the two systems disagree about the gallon in one direction and about the fluid ounce in the other, and an implementation that “corrects” for imperial by scaling the whole family uniformly gets the fluid ounce exactly backwards.
This converter therefore never offers an unnamed gallon, pint, quart or fluid ounce. Each is labelled US or imperial in the picker and in the results, so the question “which one was my figure in?” is asked before the arithmetic rather than discovered afterwards.
US and imperial, side by side
Every value below is exact by definition rather than measured, so these are the numbers themselves and not a rounding of them.
The four volume units that share a name across the two systems| Unit | US customary | Imperial (UK) | Difference |
|---|
| gallon | 3.785 411 784 L | 4.546 09 L | imperial is 20.095% larger |
|---|
| quart | 946.352 946 mL | 1 136.5225 mL | imperial is 20.095% larger |
|---|
| pint | 473.176 473 mL | 568.261 25 mL | imperial is 20.095% larger |
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| fluid ounce | 29.573 529 5625 mL | 28.413 0625 mL | US is 4.084% larger — the reversal |
|---|
A practical rule for reading a source: if the document is American, a gallon is 3.785 L; if it is British, Irish or from a Commonwealth country still using imperial for fuel or beer, it is 4.546 L. If the document is Canadian, check the date and the subject — Canada uses litres for fuel but imperial pints appear in older material and in pubs.
Cooking measures, and why they drift
Kitchen volumes are a family of conventions rather than a standard. The US customary cup is 236.588 mL, being a sixteenth of a US gallon. The US legal cup, used on nutrition labels, is 240 mL. A metric cup in Australia and New Zealand is 250 mL. An imperial cup, where it appears at all, is 284 mL. The tool uses the customary cup, because that is what US recipes mean, and labels it as such.
The gap matters less than it looks for most cooking, because a recipe is internally consistent: if every measure in it is a US cup, using US cups throughout gives the intended ratios. It matters a great deal when a single ingredient is converted in isolation, or when baking, where ratios of flour to liquid are the recipe.
What this page will not do is convert a cup of flour into grams. That is not a volume conversion at all — it depends on the density of the specific ingredient, and on how it was packed into the cup, which is why the same cup of flour is quoted anywhere from 120 g to 145 g. A tool that answers it has to ask which ingredient, and this one does not ask.
Capacity or cubic?
Volume units come in two flavours that convert freely but are used for different things. Capacity units — litres, gallons, pints — describe how much a container holds. Cubic units — cubic metres, cubic feet, cubic yards — describe a space measured by its dimensions. A litre is exactly one cubic decimetre, so the two families are the same dimension and the conversion is exact; the difference is entirely in what you are likely to be doing.
Cubic yards are the unit builders' merchants sell aggregate, topsoil and ready-mix concrete in across the US, and a cubic yard is exactly 27 cubic feet — a whole number, because 3³ is 27. In metric markets the same materials are sold by the cubic metre, which is about 1.31 cubic yards. Getting that one wrong is expensive rather than merely embarrassing, because material is ordered in whole loads.
Suppliers frequently quote aggregate by weight rather than volume, and that crossing needs a density — a cubic yard of dry gravel is roughly 1.4 short tons, but the figure moves with the material, its grading and how wet it is. That is a different question from this page, and it belongs on a tool that asks what the material is.
Doing it in a spreadsheet
Excel and Google Sheets both have a CONVERT function, and it does know the difference between the two gallons — but the unit codes are easy to misread. =CONVERT(1,"gal","l") gives the US gallon at 3.785; the imperial gallon is "uk_gal", and"uk_pt" is the imperial pint against "pt"for the US one. Reaching for the unprefixed code and getting the US value is the single most common spreadsheet volume error in a British workbook.
If you would rather not depend on those codes, multiply by the defining factor directly: =A1*3.785411784 for US gallons to litres and =A1*4.54609 for imperial. Both are exact, so nothing is lost by writing them out, and the formula then says which system it means.
Sources and methodology
Nothing on this page is fetched and there is no data feed behind it: every figure is computed from what you type, using factors that are fixed by statute or treaty. The arithmetic needs no authority; the definitions do, and that is what the sources below settle — the US gallon as 231 cubic inches, the imperial gallon as 4.546 09 litres, the litre as a cubic decimetre, and the labelling cup that differs from the customary one.