Volume is where unit conversion becomes genuinely dangerous, because two different systems use identical names for different amounts. A US gallon is 3.785 litres. An imperial gallon is 4.546 litres. That is a 20 per cent difference hiding behind the same word, and it is the single most expensive conversion error in everyday use.
Always check which gallon. US fuel economy in miles per gallon looks worse than the UK figure for the identical car, purely because the imperial gallon is larger. 30 mpg (US) is 36 mpg (imperial) — same vehicle, same fuel, different gallon.
Quick reference
The table follows whichever units you select above.
The pint problem
A US pint is 473 ml. An imperial pint is 568 ml. The imperial pint is about 20 per cent larger, which is why a pint of beer in London is noticeably more than a pint in New York.
It gets worse. A US pint is 16 US fluid ounces; an imperial pint is 20 imperial fluid ounces. And the fluid ounces themselves differ: 29.57 ml for the US, 28.41 ml for imperial. So the imperial fluid ounce is smaller while the imperial pint is larger. There is no way to hold all this in your head, which is what the converter above is for.
Cups are the worst offender
For a side-by-side reference you can keep beside a recipe, use the printable kitchen conversion chart. It separates US customary, metric/Australian and imperial measures instead of treating every cup or fluid ounce as interchangeable.
| Cup | Volume |
|---|---|
| US customary cup | 236.6 ml |
| US legal cup (nutrition labels) | 240 ml |
| Metric cup (UK, AU, NZ, CA) | 250 ml |
| Japanese cup | 200 ml |
An Australian recipe calling for a cup means 250 ml; an American one means about 236.6 ml. On a single cup that is under a tablespoon and rarely matters. Across four cups of liquid in a batter it is most of a quarter cup, and it does.
Australian tablespoons compound this: they hold 20 ml against 15 ml almost everywhere else. An Australian recipe asking for three tablespoons of something potent — baking soda, salt, chilli — is asking for a third more than a US cook would measure.
Cups of solids are not a volume problem
Converting a cup of flour to grams is not really unit conversion, because it depends on density and on how the cup was filled. Scooped flour packs to roughly 145 g; spooned and levelled it is nearer 120 g. That 20 per cent spread is why serious baking recipes are written by weight.
Common questions
How many millilitres in a US cup?
236.6 ml for the customary cup used in recipes, or 240 ml for the legal cup used on nutrition labels.
How many ounces in a litre?
About 33.8 US fluid ounces, or about 35.2 imperial fluid ounces.
Is a UK gallon bigger than a US gallon?
Yes, by about 20 per cent. The imperial gallon is 4.54609 litres against 3.785412 litres for the US gallon.