A cup is not one size. In the table above, the Taiwan rice cup is the smallest at about 180 mL, followed by the Japan cup at 200 mL, the old UK cup at roughly 227 mL, the US cup at about 236.6 mL and the metric cup at 250 mL. The biggest holds nearly 40% more than the smallest, so knowing which cup a recipe means is the first step in converting it.
Where each size comes from
The US cup is the only one here derived from a legal system of units. NIST Special Publication 1038 defines the US gallon as 231 cubic inches and the inch as exactly 2.54 cm; a fluid ounce is 1/128 of that gallon and a cup is 8 fluid ounces, which gives 236.5882365 mL. American food labels use a rounder number: under the FDA’s nutrition labelling rule, a cup on a label means 240 mL. Both are exact within their own context, which is why the site lists the “US legal cup” separately.
The metric cup of 250 mL is used in Australian recipes and measuring sets, as the taste.com.au measurement charts show, and NIST also gives 250 mL as the practical metric equivalent of a cup. The Japan cup of 200 mL has a different history. According to Kagawa Nutrition University, Aya Kagawa introduced the 200 mL cup from 1948 as a standard for teaching home cooking, and the university still sells measuring sets built on it. It is a cooking convention, not a legal unit, but it became the norm in Japanese recipes.
The rice cup comes from an older measure. Tiger, a rice cooker maker, states that one level cup is one gō (合), 180 mL, and the cups supplied with cookers in Taiwan follow the same customary size. The UK cup is the vaguest: British law defines the imperial pint and fluid ounce but never a cup. Writing in The Spectator in 1963, Elizabeth David described the breakfast cup of older British recipes as holding about 7 to 8 fluid ounces. This site uses 8 imperial fluid ounces, 227.3 mL, while some converters use half a pint, 284 mL.
How to tell which one a recipe means
Start with where the recipe was published. An American blog or cookbook means the US cup; an Australian magazine or website means 250 mL; a Japanese cookbook written for home cooks means 200 mL, often written as 1カップ. A British recipe from the last few decades probably uses grams and millilitres, and one that mentions cups is often borrowing American measures. An older British recipe that says “a cup” is the one case where no conversion is reliable.
The equipment gives clues too. A measuring cup with both cup and mL markings shows its standard: 250 mL at the 1 cup line means metric, about 240 mL means US. The small plastic cup packed with a rice cooker is almost always the 180 mL rice cup, and it should be kept for rice. When a recipe lists grams next to cups, use the grams and skip the question entirely.
When the difference matters
Take an Australian cake that calls for 2 cups of all-purpose flour. The author meant 2 × 250 mL, about 264 g. Measured with US cups, 2 × 236.6 mL gives about 250 g, around 14 g short, which is a small but real change in a cake. Measured with Japan cups it would be about 211 g, more than 50 g short, and the batter would come out noticeably thinner.
For a soup that uses 4 cups of stock, the same mix-up changes the liquid by about 54 mL (US cups instead of metric) up to 200 mL (Japan cups instead of metric), and you would simply cook it a little longer or add water. Cups matter most in baking, where flour, sugar, fat and liquid have to stay in proportion, and least in dishes you season and adjust by taste as you go.