US Cups vs Metric Cups: What’s the Difference?

One word, several sizes

A cup sounds like a fixed amount, but it is a name shared by several different volumes. The three you are most likely to meet are the US customary cup at 236.588 mL, the metric cup at 250 mL, and the Japanese cup at 200 mL. A fourth, the US “legal” cup, is 240 mL and exists for nutrition labels.

The differences look small on paper but add up quickly across a recipe. They are a plausible reason for a dish turning out too dry, too wet or too dense when it is made with measures from another tradition, especially in baking, where proportions matter.

The sections below take each cup in turn, compare them, and finish with a practical way to convert a recipe.

The US customary cup

The US cup is half a US pint, or 8 US fluid ounces. Since the US gallon is exactly 231 cubic inches (3.785411784 litres), the cup works out to exactly 236.5882365 mL, usually rounded to 236.6 mL or 237 mL.

The 240 mL “legal” cup is a different, rounded figure. US federal guidance for nutrition labelling states that a cup means 240 mL, so serving sizes on packaged food are expressed on that basis. It is a labelling convention, and it is about 1.4 per cent larger than the customary cup: 240 ÷ 236.5882 = 1.0144.

The metric cup

Australia, New Zealand and some other countries use a metric cup of 250 mL, a quarter of a litre. Older Canadian recipes may use a smaller cup of about 227 mL, which was 8 imperial fluid ounces.

The metric cup is larger than the US cup by 250 − 236.5882 = 13.41 mL, or about 5.7 per cent. In kitchen terms it is roughly one US cup plus one tablespoon.

Here is how that adds up. Three metric cups of milk is 750 mL. Three US cups is 709.76 mL. The difference is about 40 mL, nearly 2¾ US tablespoons — easy to overlook in a soup, but a real change to a batter.

The Japanese cup

In Japan the standard cup in cooking is 200 mL. It is considerably smaller than either of the others: 200 ÷ 236.5882 = 0.845 of a US cup, roughly 15 per cent less. It is also about 13½ US tablespoons (200 ÷ 14.787 = 13.5).

A recipe translated from Japanese may therefore give a cup amount that is quite different from what an American cup would hold. If a recipe states its cup size or gives a millilitre figure beside it, use the millilitres.

A quick comparison

• Japanese cup: 200 mL, about 0.85 US cup
• Older Canadian cup: about 227 mL, about 0.96 US cup
• US customary cup: 236.588 mL
• US legal cup (labels): 240 mL, about 1.01 US cups
• Metric cup: 250 mL, about 1.06 US cups

Spoons vary too. The tablespoon is 15 mL in most countries that use metric spoons, but 20 mL in Australia, where a tablespoon holds four teaspoons instead of three. A recipe from Australia that asks for 2 tablespoons therefore contains 40 mL, not 30 mL.

Why the difference matters

Two things are going on at once. The first is the cup size itself, covered above. The second is that a cup is a measure of volume, so the amount of flour or sugar in it depends on how it was scooped, sifted or packed. A single baking chart lists 1 cup of all-purpose flour as 120 g, but other sources give other numbers. A 5.7 per cent difference in cup size sits on top of that uncertainty.

For pourable liquids the cup size is the only issue, and a conversion through millilitres is exact. For dry ingredients, weighing in grams is more repeatable than any cup, which is why many bakers prefer scales.

When a recipe was written for a cup you do not own, the safest route is whatever the author gave in millilitres or grams. Failing that, convert each quantity once, write the new amounts down, and use them consistently for the whole recipe.

Converting a recipe in practice

First find out which system the recipe uses. If it lists millilitres next to cups, use the millilitres. If not, the author’s country is the best clue.

A converter can help here, even though it lists the US cup rather than a metric one. To convert metric cups, multiply by 250 to get millilitres, enter that, and convert to US cups. For example, 2 metric cups is 500 mL, which is 2.11 US cups. Two US cups is 473.2 mL.

The same route works for the Japanese cup. A recipe needing 1½ Japanese cups of stock is 300 mL, which is 1.27 US cups, or a little over 1¼ cups. Going the other way, 2 US cups is 473.2 mL, or 2.37 Japanese cups. Rounding liquids to the nearest 5 or 10 mL is almost always enough, and the converter’s decimal-places setting keeps the output tidy.

For forgiving dishes such as soups, stews and sauces, rounding is fine. For baking, use one cup for every ingredient in the recipe and do not mix systems partway through.