A cup is not a cup: US, UK, metric and Australian cups compared
A US cup is 236.6 ml, a metric cup is 250 ml, and an old UK cup is 284 ml. Why the same recipe measures differently by country, and when the gap actually matters.
Published
An American recipe says one cup of milk. Your measuring cup — bought in Europe or Australia — holds 250 ml. The recipe author’s cup held 236.6 ml. You are now 6% over on every liquid in the recipe, and nobody made a mistake.
“Cup” is not one unit. It is at least four, and they do not agree.
The cups in circulation
| Cup | Volume | Where |
|---|---|---|
| US legal cup | 240 ml | US nutrition labels |
| US customary cup | 236.6 ml | US recipes and measuring cups |
| Metric cup | 250 ml | Australia, NZ, Canada, modern UK |
| Imperial cup | 284 ml | Old UK cookbooks (half an imperial pint) |
| Japanese cup | 200 ml | Japanese recipes and rice cookers |
The US customary cup is defined by the chain: 1 US gallon = 3.785 litres, a gallon is 16 cups, so a cup is 236.588 ml. The rounder 240 ml “legal cup” exists because US food-labelling regulations wanted a tidier number. In practice US measuring cups are sold as 236.6 ml but recipes tolerate either.
The metric cup is exactly 250 ml — a quarter litre — adopted by countries that metricated and wanted cup-based recipes to survive. The old imperial cup of 284 ml turns up in pre-1970s British cookbooks and is far enough from everything else to genuinely spoil a recipe.
When the difference matters
The gap from US to metric is about 5.7%. Whether that matters depends entirely on what you are making:
Usually harmless: soups, stews, sauces, casseroles, marinades — anything you would taste and adjust anyway. Cooking is tolerant; use whatever cup you own.
Occasionally noticeable: rice and grains (the water-to-grain ratio shifts), pancake batters (consistency changes visibly, but is easy to correct by eye).
Where it bites: baking. Bread, cakes and pastry depend on ratios of flour to liquid to fat. A 6% error on each of three ingredients does not cancel out — it compounds, because you are usually over on some and using weight for others. Combine a metric cup with an American flour measurement and a cake can go from tender to dense. If a baking recipe fails “mysteriously”, a cup mismatch is one of the first things to rule out.
The deeper problem: cups measure volume, ingredients vary by density
Even with the right cup, “one cup of flour” is ambiguous in a way “125 grams of flour” never is. Flour compacts: a scooped-and-packed cup can hold 30% more flour than a spooned-and-levelled one — a far bigger error than any difference between national cup sizes. This is its own topic, covered in why cups-to-grams depends on the ingredient, and it is the reason serious baking recipes give weights.
For liquids, none of that applies — water is water however you pour it — which is why cup-size differences are the only error source for liquids, and density is the bigger one for everything dry.
Practical rules
- Identify the recipe’s origin before converting. US site → 236.6 ml (240 is fine). Australian or modern British → 250 ml. Wartime-era British cookbook → 284 ml, and treat it with care.
- For liquids, convert cups to millilitres once and measure in ml — every measuring jug on earth has a ml scale. The kitchen converter does cups, ml, dl, tablespoons and the rest in one place.
- For baking, convert to grams where the recipe offers weights, and use the ingredient-specific conversion where it does not — a cup of flour and a cup of sugar differ by nearly 80 grams.
- Within one recipe, stay consistent. A single cup used for everything keeps the ratios nearly right even if the absolute quantities drift — which is usually what the dish actually depends on.