1 Ounces to Cups

One fluid ounce is 0.125 cup, and 8 fluid ounces make a cup

One US fluid ounce is 0.125 cup, which is 1/8 of a cup, and 8 fluid ounces fill one full US cup. The 8 is a definition rather than a measurement: the NIST Handbook 44, Appendix C lists 1 cup, measuring as 8 fluid ounces (exactly) and 237 milliliters, and the 237 is the exact metric value rounded to a whole number.

Multiply out and the decimals return. One US fluid ounce is 29.5735295625 mL, fixed by the 231 cubic inch gallon and the exact 16.387064 mL cubic inch. Eight of those ounces make 236.5882365 mL. A fluid ounce is also 2 tablespoons and 6 teaspoons, so one cup is 16 tablespoons and 48 teaspoons.

Every row below the first is that 0.125 multiplied by a count: 12 x 0.125 = 1.5 cups, and 16 x 0.125 = 2 cups, which is also a US pint.

Fluid ounces measure volume, ounces measure weight

An ounce on a kitchen scale and a fluid ounce on a measuring cup share a name and nothing else. The fluid ounce measures volume, 29.5735 mL of space. The ounce measures mass, 28.3495 g, and the two meet only when you know what the stuff is.

Water is the case where they nearly match. A fluid ounce of water weighs 29.5735 g, or 1.0432 ounces on a scale, so a cup of water weighs 8.35 ounces and not 8. Milk runs heavier at 8.61 ounces per cup, and all-purpose flour runs far lighter at 4.41.

Nutrition labels make the split official. 21 CFR 101.9(b)(5)(viii) says that for labeling purposes 1 fl oz means 30 mL and 1 oz in weight means 28 g, so the two numbers on a panel are separate conventions rather than a conversion. The FDA guidance on metric equivalents repeats the same rule for household measures.

The test that settles it: if a recipe says 8 ounces of milk, a scale reads 8 and the cup reads about 0.93, while 8 fluid ounces reads 1 cup and 8.6 ounces.

Worked examples from 1 to 16.9 fluid ounces

Six amounts cover most of what a recipe or a label asks for, and each one comes from the same 0.125 multiplier.

1 fluid ounce is 0.125 cup, which is 2 tablespoons, 6 teaspoons and 29.5735 mL. It is the small line near the bottom of a liquid measuring cup, and one shot of espresso.

4 fluid ounces is 0.5 cup, or 118.2941 mL, and 8 tablespoons, about a small juice glass.

8 fluid ounces is 1 cup, or 236.5882 mL, the number every other row is built from.

12 fluid ounces is 1.5 cups, or 354.8824 mL, the standard soda can. 16 fluid ounces is 2 cups, or 473.1765 mL, which is a US pint.

The one that catches people is 16.9 fluid ounces, the water bottle sold everywhere. Divide by 8 and it is 2.1125 cups, and 16.9 x 29.5735295625 = 499.79 mL. That bottle is a 500 mL bottle with a fluid ounce label, so it holds a little more than two cups and a little less than a pint.

The 240 mL label ounce and the 236.588 mL kitchen ounce

A cereal box prints 1 cup (240 mL), and a measuring cup holds 236.5882 mL. Both are correct, because the label cup is a separate unit defined for nutrition panels.

21 CFR 101.9 sets a teaspoon at 5 mL, a tablespoon at 15 mL, a cup at 240 mL and a fluid ounce at 30 mL. Those round numbers keep panel arithmetic easy: eight label fluid ounces of 30 mL make exactly 240 mL. Against the kitchen cup the same ounce runs 30 / 29.5735295625 = 1.0144, or 1.44 percent larger, which puts a label cup 3.4118 mL above a US cup.

The gap is worth about two thirds of a teaspoon per cup, so it never ruins a cake. It matters when you count servings: eight panel cups are 1,920 mL, while eight kitchen cups are 1,892.7 mL.

Tablespoons, teaspoons and the fractions of a cup

Fluid ounces divide into tablespoons and teaspoons cleanly, which is why American recipes move between the three without a chart. A tablespoon is half a fluid ounce, so 1 fluid ounce is 2 tablespoons. A teaspoon is one sixth of a fluid ounce, so 1 fluid ounce is 6 teaspoons and a cup is 48.

The thirds are the awkward rows, because 8 divides by 2 and by 4 but not by 3. A third of a cup is 2.667 fluid ounces, which is 5 tablespoons plus 1 teaspoon, and two thirds is 10 tablespoons plus 2 teaspoons. Every other row in the table is exact.

Halving a recipe compounds the fractions: half of 2/3 cup is 1/3 cup, and half of 1/3 cup is 2 tablespoons plus 2 teaspoons.

What a fluid ounce weighs in flour, sugar and milk

A fluid ounce of flour weighs about 0.55 ounce, and a fluid ounce of honey weighs about 1.5. The volume is the same in both cases and the weight is not, because the two ingredients pack different mass into the same 29.5735 mL.

A cup of flour weighs 125 g, so 125 / 8 = 15.625 g per fluid ounce, and 15.625 / 28.3495 = 0.5512 ounces. A cup of sugar at 200 g gives 25 g per fluid ounce, or 0.8818 ounces.

Butter is the ingredient that lands on a round number. A US stick is 1/2 cup and weighs 4 ounces, so a cup of butter is 2 sticks and 8 ounces, and the wrapper markings mean you never need the table. For flour and sugar, a scale is the only tool that repeats from one cup to the next, because a scooped cup can hold 20 percent more than a spooned one.

Imperial fluid ounces and the 4 percent shortfall

British and Australian recipes use fluid ounces that are not American fluid ounces, and the gap is large enough to change a bake. The imperial fluid ounce is 28.4130625 mL, one twentieth of the imperial pint of 0.56826125 liters fixed by the Weights and Measures Act 1985.

Convert it and an 8 fluid ounce measure from a UK book is 227.3045 mL, which is 0.9608 of a US cup. That is 3.9 percent short of the cup an American cook would scoop, and on a four cup batter it is 37 mL missing. The same book may also print an imperial cup of 284.130625 mL, which is 10 imperial fluid ounces and 1.2009 US cups, a 20 percent overshoot in the other direction.

Neither number appears on modern packaging: a UK carton prints milliliters and a US carton prints fluid ounces, so work in milliliters whenever a recipe crosses a border.

Metric cups add a third size at 250 mL, 5.7 percent above a US cup. A recipe from Sydney that lists 1 cup means 250 mL, and four cups of stock would be 1,000 mL rather than the 946.4 mL an American cook would pour.

Reading a bottle or a can in cups

Bottles and cans are where the fluid ounce to cup conversion earns its keep, because the label is in ounces and the recipe is in cups.

A quart of milk is 4 cups, which is the conversion that saves the most trips to the store. A half gallon is 8 cups and a gallon is 16, so a recipe calling for 6 cups of milk needs a half gallon plus a little of a second carton, not a gallon.

Coffee is where the numbers get loose. A mug sold as 12 ounces holds 12 fluid ounces to the brim and about 10 in use, while a travel mug rated 16 holds about 14. Diet advice adds its own wrinkle: eight cups a day means 64 fluid ounces, or 1.89 liters, while the same sentence read as eight 250 mL metric cups means 2 liters.