Convert cups to fluid ounces

One US cup holds exactly 8 fluid ounces. The American measurement institute NIST prints that equality as the definition of the cup and marks it as exact[1].

The cup is a unit of volume used in American recipes and it is measured with a vessel of fixed capacity. The fluid ounce (fl oz) measures the same quantity and holds 29.5735 millilitres. The ounce on a scale carries the same word and measures mass instead — the ounces to grams converter handles that one.

The labelling cup holds exactly 240 millilitres[2], which comes to 8.12 fluid ounces. The metric cup holds exactly 250 millilitres, which comes to 8.45 fluid ounces. You pick the measure with the Cup type switcher under the converter fields.

The converter starts on the US customary cup, because that is what a recipe and a measuring cup sold in the United States both mean. The reverse calculation is handled by the ounces to cups converter.

The whole American series of volume measures is built on the gallon, and the gallon has a fixed size: exactly 231 cubic inches[1]. An inch is exactly 2.54 centimetres, so that arithmetic puts the gallon at 3.785411784 litres.

The gallon splits into 128 fluid ounces and, separately, into 16 cups. Put those two divisions together and the figure eight follows. One hundred and twenty-eight ounces divided by sixteen cups gives eight ounces per cup.

The same arithmetic gives the cup in millilitres. Divide the gallon of 3,785.411784 millilitres by sixteen and you get 236.5882365 millilitres, which is what an American measuring cup holds. The cups to ml converter does that calculation in full.

The same series has three measures larger than the cup. A US pint holds two cups and 16 ounces. A quart holds four cups and 32 ounces. A gallon holds sixteen cups and 128 ounces.

The same series also has measures smaller than the cup. A US tablespoon holds half a fluid ounce and a teaspoon holds a sixth of one.

Multiply the number of cups by eight.

The formula is fluid ounces = cups × 8 for the US customary cup. The labelling cup uses 8.1154 and the metric cup 8.4535.

American recipes state volume in fractions, because a measuring set is sold in exactly those sizes: 1/4, 1/3, 1/2 and 1 cup, with 2/3 and 3/4 marked on the graduations[3]. The converter field therefore takes a fraction straight from the recipe.

Type 3/4 and you get 6 ounces. Type 2 1/2, with a space between the whole number and the fraction, and you get 20 ounces. Decimals work the same way, and the comma and the full stop are interchangeable, so 2.5 and 2,5 give the same answer.

Three fractions land on round numbers under the US customary cup. Half a cup is 4 ounces. A quarter cup is 2 ounces. An eighth of a cup is one ounce.

A volume written in millilitres is handled by the ml to ounces converter, and a volume written in tablespoons by the tablespoons to ml converter.

A cup filled with water holds 8 fluid ounces, and the water inside it weighs about 8.35 ounces. Both figures describe the same glass: the first states volume, the second states mass.

The two numbers sit close together because of density. Water weighs roughly one gram per millilitre and a fluid ounce holds 29.57 millilitres, so a fluid ounce of water weighs about 29.6 grams. An ounce on the scale weighs 28.349523125 grams[6], which is 1.22 grams less, and that gap turns eight ounces of volume into 8.35 ounces of weight.

Water, milk and juice all have a density close to one gram per millilitre, so their two figures stay close. Dry goods have a lower density, because air sits between the grains, and the same cup then carries less mass.

A recipe that gives a liquid in ounces means fluid ounces, and a measuring jug answers it. A recipe that gives cheese or chocolate in ounces means mass, and a kitchen scale answers that one.