How a Cup Is Actually Defined
There is no single international cup. A U.S. recipe cup, a metric cup and a British cup are
three different volumes, which is why a converter is worth more than a conversion chart.
For food sold in the United States there is one authoritative set of definitions: the
nutrition-labelling rule spells out the household measures in metric terms, and those
rounded numbers are what appears on every U.S. nutrition label and in every recipe that has
been checked against them.
Household measures as defined for U.S. nutrition labelling, 21 CFR 101.9(b)(5)(viii).
| Measure | Label definition | Equals |
| Teaspoon | 5 mL | The base unit for small measures; three make a tablespoon. |
| Tablespoon | 15 mL | Three teaspoons. |
| Cup | 240 mL | Sixteen tablespoons, or eight labelled fluid ounces. |
| Fluid ounce | 30 mL | Two tablespoons. |
| Ounce in weight | 28 g | The weight ounce, not the fluid ounce — the two are not interchangeable. |
Those labelling values are deliberately rounded, and it is worth knowing by how much. Eight
fluid ounces of the customary U.S. fluid ounce (29.5735 mL) come to 236.6 mL, so the
labelled 240 mL cup is about 1.4 per cent larger than the cup you would pour from a
graduated jug. That gap never matters for a single cake and it does matter when a
commercial recipe is scaled by a hundred. Where the number has to be exact, weigh the
ingredient instead — a cup is a volume, and the same volume of two powders weighs two
different amounts.
What one cup actually weighs
This is the table the flour-conversion questions all come down to. Fill a cup by scooping
straight from the bag and you compact the flour and add on the order of ten to twenty per
cent more of it; spoon it in and level it and you get the published figure. Both methods
are called "one cup", and the recipes that work in grams are the ones that avoid the
argument.
Published one-cup weights (King Arthur Baking ingredient weight chart; figures listed
by the chart in cups, half cups or quarter cups are stated per cup here).
| Ingredient | 1 cup weighs | Per tablespoon |
| All-purpose flour | 120 g (4¼ oz) | About 7.5 g |
| Bread flour | 120 g (4¼ oz) | About 7.5 g |
| Cake flour, unbleached | 120 g (4¼ oz) | About 7.5 g |
| Whole wheat flour | 113 g (4 oz) | About 7 g |
| Self-rising flour | 113 g (4 oz) | About 7 g |
| Cornstarch | 112 g (4 oz) | About 7 g |
| Sugar (granulated white) | 198 g (7 oz) | About 12 g |
| Brown sugar, packed | 213 g (7½ oz) | About 13 g |
| Confectioners' sugar, unsifted | 113 g (4 oz) | About 7 g |
| Cocoa, unsweetened | 84 g (listed at 42 g per ½ cup) | About 5 g |
| Butter | 227 g (8 oz; listed at 113 g per ½ cup) | About 14 g |
| Water or milk | 227 g (8 oz) | About 14 g |
| Honey | 336 g (listed at 21 g per tablespoon) | 21 g |
| Table salt | 288 g (listed at 18 g per tablespoon) | 18 g |
| Rolled oats | 89 g (3⅛ oz) | About 5.5 g |
| Cornmeal, whole | 138 g (4⅝ oz) | About 8.6 g |
Worked conversions for recipe-sized amounts
- 2¾ cups all-purpose flour in grams? 2.75 × 120 g = 330 g.
The same arithmetic answers the reverse question: 330 g of all-purpose flour is 2¾
cups, and 330 g of cake flour is also 2¾ cups, because the two flours share the same
published weight per cup.
- 3 cups of cake flour? 3 × 120 g = 360 g. If the cake flour has
to be built from all-purpose plus cornstarch instead, use the measured blend rather than
arithmetic guessing — the
flour
substitution calculator carries the published ratio and scales it for you.
- 1½ cups whole wheat flour? 1.5 × 113 g = 170 g — note
that this is 10 g less than the same 1½ cups of all-purpose, because whole wheat
weighs less per cup.
- 2 cups of granulated sugar? 2 × 198 g = 396 g. Sugar is denser
than flour by a wide margin, which is why a cup of it weighs about 65 per cent more.
- 20 cups of plain flour for a bulk batch? 20 × 120 g = 2,400 g,
or 2.4 kg. At that size the labelling cup and the customary cup finally separate: 20
labelled cups is 4.8 L, while 20 customary cups is 4.73 L.
One honest caveat about combining the two halves of this page. The labelling rule rounds a
cup to 240 mL for convenience, while the published one-cup weights are measured in a
customary cup of 236.6 mL. Multiplying 240 mL by an ingredient's density therefore does not
reproduce the weight chart exactly, and it is the chart you should use — those figures
were measured on a scale, and the 240 mL definition exists to standardise nutrition labels
rather than kitchen scoops. The other trap worth naming is the difference between the two
ounces. A recipe that asks for "8 oz of chocolate" means weight, and 8 oz in weight is 227 g
of chocolate; the labelling rule puts an ounce in weight at 28 g. A recipe that asks for
"8 fl oz of milk" means volume, and that is a cup. Substituting one for the other is a
common cause of a batter that will not set.
Read the table as a set of density conversions rather than a fixed law. A cup of flour
holds 240 mL of volume whichever way you fill it; what changes is how much flour occupies
that volume. Once you have the gram weight, the
flour
substitution calculator turns it into a swap between flour types, and the
recipe scaler
applies it to a whole recipe instead of a single line.
Sources: 21 CFR 101.9(b)(5)(viii), nutrition labelling of food, on eCFR;
King Arthur Baking, ingredient weight chart.