How Many Ounces Are in a Cup?

The answer most sites give is 8. That is right exactly once: a US fluid ounce, measured in a US customary cup, holding a liquid. Change any one of those three and the number moves — to 8.12 on a food label, to 9.6 in an old British cup, or to something entirely different the moment you weigh a dry ingredient.

Flip between Fluid and Dry, then change the cup — watch "8" turn into something else.

The clean answer, stated honestly

A US customary cup is defined as exactly 8 US fluid ounces, which works out to 236.59 millilitres. So for water, milk, stock or any thin liquid measured in a standard American measuring cup, 1 cup = 8 fl oz, and the common fractions follow neatly: ¾ cup is 6 fl oz, ½ cup is 4 fl oz, ⅓ cup is about 2.7 fl oz, and ¼ cup is 2 fl oz. If that is all you needed, you are done. The rest of this page is about the three ways that "8" quietly stops being true.

Trap one: "ounce" is two different units

English uses the word ounce for two unrelated things. A fluid ounce measures volume (29.5735 mL in the US system). An ounce by weight — the avoirdupois ounce — measures mass (28.35 grams). They only line up for water-like liquids, and even then not perfectly: a cup of water is 8 fluid ounces but weighs about 8.3 ounces.

For dry ingredients the gap is enormous, because a cup is a fixed space and ingredients have wildly different densities. King Arthur Baking's ingredient weight chart puts one cup of all-purpose flour at 120 g — about 4.2 ounces by weight, barely half of "8". One cup of granulated sugar is 200 g (about 7 oz); one cup of honey is 340 g (about 12 oz). "8 ounces equals one cup" is a liquid rule that recipes wrongly stretch over everything.

Trap two: the food-label cup isn't the measuring cup

The cup printed on US Nutrition Facts panels is not the cup in your kitchen drawer. Under FDA regulation 21 CFR 101.9(b)(5)(viii), for labeling purposes a cup is defined as 240 mL, a tablespoon as 15 mL, and one fluid ounce as a round 30 mL. By that definition a labeling cup holds about 8.12 US fluid ounces, not 8 — because the customary cup is 236.59 mL and the legal cup is 240 mL, a 3.4 mL difference baked into the rules. It is small, but it is why a label's "serving" and your measuring cup never quite agree.

Trap three: not everyone's cup is your cup

The old imperial cup used across the pre-metric UK was 10 imperial fluid ounces — roughly 284 mL, or about 9.6 US fluid ounces. That is why an old British recipe's "cup" overfills an American one. Modern Australia, New Zealand and Canada use a metric cup of 250 mL, and most current UK recipes skip the cup entirely and give grams and millilitres instead. So a recipe that simply says "1 cup" is ambiguous until you know which country wrote it.

Why this wrecks recipes in practice

The failure is almost always a cross-Atlantic one. An American recipe calls for "1 cup of butter" — that means two sticks, 227 g, which happens to be about 8 ounces by weight, so butter accidentally reinforces the myth. Then the same recipe says "1 cup of flour," a cook assumes 8 ounces, weighs out 227 g instead of 120 g, and the bake is built on nearly double the flour. The fix every serious baker reaches for is the same one: stop converting cups to ounces and weigh in grams, where a number means exactly one thing.

A cup was never a unit of weight — so "how many ounces in a cup" only has a single clean answer for the one liquid, in the one country, that the question quietly assumes.