US cup vs metric cup: the size difference that matters

“One cup” means three different volumes depending on where the recipe was written, and none of the three agree. The US cup is exactly 236.5882365 mL. The metric cup, used in Australia, New Zealand, and some other countries, is a clean 250 mL. There's no international standard “cup” unit — each is a regional convenience measure, and mixing them up is one of the most common reasons an imported recipe doesn't turn out right.

The three cups, side by side

The US figure is exact because it derives from the US gallon (231 cubic inches, legally defined). The metric cup isn't derived from a base unit fraction at all — it's simply a round number chosen for convenience once metric measuring cups became standard kitchenware. Japan's rice-cooker cup is a separate convention entirely, unrelated to either.

How the 5.7% gap compounds

250 ÷ 236.5882365 ≈ 1.0567, so a metric cup holds about 5.7% more than a US cup. On a single cup of milk in a savory dish, that's invisible. On flour in a cake recipe scaled to 3–4 cups, a consistent 5.7% error across every ingredient shifts the whole ratio of dry to wet ingredients, not just the total volume — which is why the same recipe can turn out drier or denser depending on which cup the cook actually used.

Where the metric cup came from

Unlike the US cup, which is a fixed fraction of the legally defined US gallon, the 250 mL metric cup has no equivalent chain back to an SI base unit — it's simply a convenient round number chosen once countries like Australia adopted the metric system for everyday cooking measures in the 1970s. That's also why the metric cup divides so cleanly into familiar kitchen fractions: a half metric cup is exactly 125 mL, a quarter is exactly 62.5 mL, both round numbers a US cup can't match, since a quarter US cup is 59.147 mL, an inherently less tidy figure.

How to tell which cup a recipe means

A few tells help identify the source:

• Site or publisher origin. A US food blog, US cookbook, or US brand's packaging almost always means the 236.588 mL cup, even if it doesn't say so explicitly.
• Explicit mL equivalents.Some recipe sites print “1 cup (250 mL)” in parentheses — that parenthetical is the giveaway for a metric-cup recipe.
• Weight instead of volume. Any recipe that gives grams for dry ingredients sidesteps the ambiguity entirely — weight doesn't care which cup was used to fill it, which is why professional and European bakers overwhelmingly measure flour and sugar by mass rather than volume.

When you do need to convert, use the exact factor rather than rounding both cups to “about the same” — a US cups to milliliters converter applies 236.5882365 precisely, which is the number to scale against a 250 mL metric-cup recipe if you want a faithful conversion rather than an approximate one.