"One cup" sounds like it should be unambiguous. It is not. Several cup standards are in active use, and the gap between the largest and smallest is big enough to ruin a cake.
The standards you will actually meet
| Standard | Volume | Where you find it |
|---|---|---|
| US customary cup | 236.6 ml | American recipes, most English-language food blogs |
| US legal cup | 240 ml | US nutrition labelling |
| Metric cup | 250 ml | Australia, New Zealand, much of Europe |
| Imperial cup | 284.1 ml | Older British cookbooks |
The US customary cup is the one this site uses, and the one the overwhelming majority of recipes written in English assume.
How much this actually matters
A metric cup is about 6% larger than a US cup. On one cup of flour that is roughly 7 grams — genuinely negligible.
The problem is accumulation. Take a cake using four cups of flour, two of sugar and one of milk, and measure it all with metric cups when the recipe meant US cups. You add about 28 grams of extra flour, 24 grams of extra sugar and 15 grams of extra milk. The individual errors are small; the combined shift in the flour-to-liquid ratio is enough to change the crumb.
The imperial cup is the one to watch properly. At 284 ml it is 20% larger than a US cup. Following an old British recipe with American cups — or the reverse — will not produce a slightly different cake. It will produce a failed one.
Spoons vary too
This catches people out more often than cups, because nobody expects it:
- A US tablespoon is 14.8 ml — exactly 1/16 of a US cup, and 3 teaspoons.
- An Australian tablespoon is 20 ml — four teaspoons, not three.
- A teaspoon is 4.9 ml almost everywhere, which is the one piece of good news.
An Australian recipe calling for two tablespoons of baking powder means 40 ml. Measured with US tablespoons you would use 30 ml — a 25% shortfall on a leavening agent, which is exactly the kind of ingredient where that matters.
Working out which one a recipe means
Recipes almost never label their cup standard, so use context:
- American sites, US cookbooks, most food blogs — US customary cup.
- Australian and New Zealand recipes — 250 ml metric cup, and 20 ml tablespoons.
- Modern British recipes — usually grams and millilitres, so cups appear rarely.
- Older British cookbooks — possibly the imperial cup. Treat any pre-1970s British recipe using cups with suspicion.
- If a recipe gives both cups and grams — use the grams. They are unambiguous and they are what the recipe was tested with.
Dry cups and liquid cups are the same volume
A common misconception worth clearing up: a dry measuring cup and a liquid measuring jug hold the same volume. They are shaped differently for practical reasons — you level a dry cup with a straight edge, and you read a liquid jug at eye level against a marked line, which needs headroom above the marking.
Using the wrong one introduces error through technique, not through capacity. Measuring flour in a liquid jug is difficult to level accurately; measuring milk in a dry cup means filling it to the brim and trying not to spill it.
Converting between cup standards
If you need to move a recipe between systems, these are the factors:
| From | To | Multiply by |
|---|---|---|
| US cup | Metric cup | 0.946 |
| Metric cup | US cup | 1.057 |
| US cup | Imperial cup | 0.833 |
| Imperial cup | US cup | 1.201 |
So an Australian recipe calling for 3 metric cups of flour needs 3 × 1.057 = 3.17 US cups. In practice, if the difference is going to matter, convert the recipe to grams instead — it is less error-prone than chaining two approximations.
Why the standards diverged
The US customary cup descends from the half-pint of the old English wine gallon, which the United States kept after independence while Britain moved to the imperial gallon in 1824. That single decision is why an imperial pint is 20 fluid ounces and a US pint is 16, and why the two cups differ by a fifth.
The metric cup is a later invention: a clean 250 ml adopted alongside metrication in Australia and New Zealand precisely because it is a round number in millilitres. It was never meant to match either older cup, which is why it sits awkwardly between them.
What we use
Every conversion on this site uses the US customary cup at 236.6 ml, and every figure is calculated from that single standard rather than mixed between sources. If you are cooking from an Australian or older British recipe, expect our numbers to run slightly small, and prefer the recipe's own gram measurements wherever it provides them.
The way around all of it
Weight has no regional variants, no legal versus customary distinction, and no 20 ml tablespoon surprise. A gram is a gram in every kitchen on earth. That is a large part of why professional and international recipes are written that way, and why our converters always show you the gram figure alongside the cup one.