A cup is not a cup: US, UK, metric and Australian cups compared

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’re now 6% over on every liquid in the recipe, and nobody made a mistake.

“Cup” isn’t one unit. It’s at least four, and they don’t 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 comes from a chain of definitions: 1 US gallon is 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 get sold as 236.6 ml and recipes tolerate either.

The metric cup is exactly 250 ml, a quarter litre, adopted by countries that metricated and wanted their cup-based recipes to survive. The old imperial cup of 284 ml turns up in pre-1970s British cookbooks, and it’s far enough from everything else to spoil a recipe outright.

When the difference matters

The gap from US to metric is about 5.7%. Whether that matters depends entirely on what you’re making.

Usually harmless: soups, stews, sauces, casseroles, marinades. Anything you’d taste and adjust anyway. Cooking is tolerant, so use whatever cup you own.

Occasionally noticeable: rice and grains, where the water-to-grain ratio shifts, and pancake batters, where the consistency changes visibly but is easy to correct by eye.

Where it bites is baking. Bread, cakes and pastry depend on ratios of flour to liquid to fat. A 6% error on each of three ingredients doesn’t cancel out. It compounds, because you’re usually over on some and using weight for others. Combine a metric cup with an American flour measurement and a cake goes from tender to dense. When 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, and a scooped-and-packed cup can hold 30% more flour than a spooned-and-levelled one. That’s a far bigger error than any difference between national cup sizes.

It’s a topic of its own, covered in why cups-to-grams depends on the ingredient, and it’s why serious baking recipes give weights.

None of that applies to liquids, since water is water however you pour it. So cup-size differences are the only error source for liquids, while density is the bigger one for everything dry.

Practical rules

• Identify the recipe’s origin before converting. US site means 236.6 ml, and 240 is fine. Australian or modern British means 250 ml. A wartime-era British cookbook means 284 ml, and treat it with care.
• For liquids, convert cups to millilitres once and measure in ml, because every measuring jug on earth has a ml scale. The kitchen converter handles cups, ml, dl, tablespoons and the rest in one place.
• For baking, convert to grams wherever the recipe offers weights, and use the ingredient-specific conversion where it doesn’t. A cup of flour and a cup of sugar differ by nearly 80 grams.
• Within one recipe, stay consistent. One cup used for everything keeps the ratios nearly right even when the absolute quantities drift, and the ratios are usually what the dish actually depends on.