One cup is 8 fluid ounces, 236.5882 milliliters and 16 tablespoons

One US cup holds 8 fluid ounces, and that number is exact. The cup is not a separate physical standard. It is a count of fluid ounces. NIST prints the relationship in its Handbook 44, Appendix C as 1 cup, measuring = 8 fluid ounces (exactly) = 237 milliliters = 1/2 liquid pint (exactly), and the 237 in that column is the exact value rounded to a whole milliliter.

Do the multiplication and the extra decimals return. One US fluid ounce is 29.5735295625 mL, fixed by the 1959 agreement on the international inch and by the 231 cubic inch gallon. Eight of those ounces make 236.5882365 mL. Sixteen tablespoons fill the same cup. Forty eight teaspoons fill it. Half a US pint, a quarter of a US quart, one sixteenth of a gallon.

The eight is the part worth remembering. Every other row is that eight multiplied by a fixed factor. Three cups is 24 fluid ounces, and 709.7647 mL in a jug.

The 240 mL cup on a nutrition label is a different unit

A cereal box says 1 cup (240 mL). That cup is not the one in your drawer. 21 CFR 101.9(b)(5)(viii) sets the household measures used for nutrition labeling: a teaspoon means 5 mL, a tablespoon means 15 mL, a cup means 240 mL, a fluid ounce means 30 mL, and an ounce in weight means 28 g. Each figure is a round number picked for easy arithmetic.

The label system is consistent with itself. Eight label fluid ounces of 30 mL make exactly 240 mL. It drifts from the kitchen in both directions at once: a label fluid ounce runs 1.4 percent above a real one, a label tablespoon runs 1.4 percent high, and a label cup ends up 3.4118 mL larger than a US measuring cup, or 1.44 percent.

That gap matters in one direction only. Counting calories, use the gram weight on the panel and stop there. Cooking from the panel, a level cup from a dry measure is close enough, because 1.4 percent of a cup is a teaspoon and a half. The FDA walks through the same convention in its guide to the Nutrition Facts label, where serving sizes pair a household measure with a metric amount.

Serving sizes also move in steps. The rule allows cups in 1/4 or 1/3 cup increments, so a panel prints 2/3 cup (160 mL) rather than 0.667 cup.

Metric cups, imperial cups and the 20 percent spread between them

Three other cups turn up in American kitchens. The metric cup is 250 mL and it is the everyday cup in Australia, New Zealand and Canada. The imperial cup is 284.130625 mL, half an imperial pint, and it survives in British cookbooks printed before the 1970s. No law defines a British cup: the Weights and Measures Act 1985 fixes the imperial pint at 0.56826125 liters and the imperial fluid ounce at one twentieth of that, which puts a half pint cup at 10 imperial fluid ounces of 28.4130625 mL each.

The metric gap bites in practice. A cake recipe written in Sydney and baked in Ohio with a US cup runs 5.7 percent short on liquid, 13.4 mL per cup, and a four cup batter loses 54 mL. That is enough to change a custard. The imperial gap is bigger and rarer, and 20 percent short is a real failure in bread.

The fix costs nothing. Look for milliliters beside the cups. A modern metric recipe prints both, and the milliliter figure is the one the author weighed. If only cups appear, check the year and the country on the page before you reach for a measure.

Dry cups and liquid cups are two different tools

A liquid measuring cup is a pitcher with a spout, a handle and lines below the rim. You fill it, set it on the counter and read the line at eye level. Its volume is calibrated to the line. A dry measuring cup is a straight sided tin that you fill and level with the back of a knife, and its volume is calibrated to the brim.

Both hold 236.5882 mL when used as designed, so a cup is a cup. The difference is what the tool lets you do. Pouring flour into a pitcher and reading the line packs the flour against the wall, and a packed cup can weigh 150 g against 125 g for a spooned and leveled cup, a 20 percent swing. Leveling a liquid in a dry cup loses a few milliliters to the meniscus.

Two habits prevent most of the trouble. Spoon the ingredient into a dry cup and sweep the top flat instead of scooping, which compacts it. Read a liquid cup on a flat surface with the line at eye level, because a cup viewed from above reads low. Neither habit changes the unit. Both change how close your cup gets to it.

A scale settles the argument. A gram reading does not care which cup you own. For flour and sugar, the scale is the only tool that repeats.

Half a cup, a third of a cup and the fractions a recipe prints

Most American recipes work in fractions, and the fractions convert cleanly because 8 divides by 2 and 4 but not by 3. Half a cup is 4 fluid ounces and 118.2941 mL. A quarter cup is 2 fluid ounces and 59.1471 mL. A third of a cup is 2.6667 fluid ounces and 78.8627 mL, which is why a third cup measure exists.

The awkward rows are the thirds. A third of a cup is 5 1/3 tablespoons, so a cook without a third cup measure uses 5 tablespoons plus a teaspoon and lands at 78.86 mL. Two thirds is 10 tablespoons plus 2 teaspoons.

Halving a recipe is where fractions compound. Half of 2/3 cup is 1/3 cup, half of 1/3 cup is 2 tablespoons plus 2 teaspoons, half of 1/4 cup is 2 tablespoons. Write the tablespoons down before you start.

Worked example: three cups of milk, and a 16.9 ounce bottle

A pancake recipe calls for 1 1/2 cups of milk and you are tripling it. Three times 1.5 cups is 4.5 cups. Multiply by 8 and you need 36 fluid ounces. Multiply by 236.5882365 and you need 1064.6471 mL, or 1.065 liters. A quart of milk is 32 fluid ounces, so you are 4 ounces short and need a second container.

The same job on a scale. One cup of whole milk weighs about 244 g, so 4.5 cups is 1098 g. Milk is 3 to 4 percent denser than water, so both figures are right: the milliliters name the carton, the grams name the weight.

The second example runs the other way. A 16.9 fluid ounce water bottle holds 16.9 / 8 = 2.1125 cups, and 16.9 x 29.5735295625 = 499.79 mL, the 500 mL bottle sold everywhere. Two of them are 4.225 cups, or 33.8 fluid ounces, a hair over a US quart.

A panel adds a third case. A frozen dinner lists 2.5 servings at 1 cup (240 mL) each. That is 600 mL on the label system, and 2.54 US cups if you scoop it with a kitchen measure. The 0.04 cup gap will never change a dinner, and it explains why a panel total and a measuring cup disagree by a few percent.

Fluid ounces back to cups, and the drinks that ignore both

The reverse direction divides by 8. Thirty two fluid ounces is 4 cups. Twelve fluid ounces is 1.5 cups. A 20 ounce soda is 2.5 cups. Divide by 29.5735295625 instead and the same amounts come back in milliliters: 946.35, 354.88 and 591.47.

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, and a travel mug rated 16 ounces often holds 14.

Diet advice uses the same word with a different size. Eight cups a day means 64 fluid ounces, or 1.89 liters. Read as 8 metric cups it would be 2 liters.

What one cup weighs when the ingredient is flour, sugar or butter

A cup is a volume. An ounce on a scale is a weight. They meet only when you know the density of what you are measuring. One cup of all-purpose flour weighs about 125 g, or 4.41 ounces. One cup of water weighs 236.6 g, or 8.35 ounces. The flour cup is 53 percent of the water cup by weight and exactly equal by volume.

Butter is the rare case 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.

Convert once, with your own cup and your own flour, and write the number inside the cupboard door. Spooned, scooped and sifted flour from one bag can differ by 20 percent. The figures above describe a spooned and leveled cup.