How many ml is a US cup? The exact answer, fast, explained

Search “how many ml in a cup” and you’ll see answers ranging from 236 to 250, and none of the popular rounded answers is technically correct. The legally exact figure for a US customary cup is 236.5882365 milliliters. It’s an exact, terminating decimal, not an approximation — the same way a mile is exactly 1.609344 km, the cup is defined as a precise fraction of the US gallon, which is itself defined in exact cubic inches.

Where 236.5882365 comes from

The US liquid gallon is legally defined as exactly 231 cubic inches, which works out to exactly 3.785411784 liters. A cup is 1/16 of a US gallon (there are 16 cups to a gallon, going cup → pint → quart → gallon by doubling). Dividing gives the exact figure:

3.785411784 L ÷ 16 = 0.2365882365 L = 236.5882365 mL

This chain of exact definitions is documented in NIST Handbook 133, the US federal reference for checking the net contents of packaged goods, which lists the cup as a standard unit of liquid measure for labeling purposes.

Reference table: US cups to mL

The “rounded” column is what most US recipe cards, appliance manuals, and nutrition labels actually print — it’s close enough for the kitchen, but it isn’t the legal or scientific figure.

Cups, tablespoons, and teaspoons all derive the same way

The cup isn't the only US volume unit built as a fraction of the gallon. A tablespoon is 1/256 of a US gallon (14.7868 mL), and a teaspoon is 1/3 of a tablespoon (4.9289 mL) — the whole US customary volume system for cooking nests inside the same 231-cubic-inch gallon definition, which is why every exact conversion between them terminates cleanly rather than needing an approximation. Sixteen tablespoons make exactly one cup, and three teaspoons make exactly one tablespoon, so working from the gallon down (or the teaspoon up) always lands on the same figures a cups-to-mL converter would produce.

Why the rounding happens

240 mL is a friendlier number to print and measure with a standard metric jug marked in 10 mL increments, so it became the de facto shorthand on US food labels and many internationally published American recipes. The gap between 236.588 and 240 is about 1.4% — small enough to disappear into normal cooking variance, but large enough to matter in precision baking or lab and pharmacy contexts where a cups-to-mL converter should use the exact factor instead.