1 Cup of Water in Milliliters

One cup of water is 240 milliliters on a label

One cup of water is 240 mL the moment that cup appears on a nutrition label. Nobody measured it with a jug. The FDA defined it in 21 CFR 101.9(b)(5)(viii), next to a 15 mL tablespoon, a 30 mL fluid ounce and a 28 g ounce. The rule exists so a bottler can print a serving size without arguing about the last decimal, and it is the reason a label never reads 236.6.

The eight cups a day advice is built on it: 1,920 mL, a shade under two liters. When a food diary records one cup of juice, this is the cup it means.

The label cup is 3.4 mL bigger than the cup in a kitchen drawer, about two thirds of a teaspoon. Nobody tastes that in a glass of water. In a salt cure, where salt is weighed against the water, it shifts the strength by 1.4 percent, and in a scone dough it moves the hydration line by the same amount.

The glass measuring cup reads 236.6 instead

A US liquid measuring cup is an 8 fl oz vessel, and here the ounces are exact. NIST Handbook 44, Appendix C, 2024 edition lists a measuring cup as 8 fluid ounces exactly and prints 237 mL beside it, a rounding to whole milliliters. The unrounded figure comes from the gallon: 231 cubic inches divided by 16 cups gives 14.4375 cubic inches, and that converts to 236.588 mL.

So the line marked 1 cup on a glass jug is 236.59 mL. Fill the same jug to 240 mL and the surface sits above that line, near where an 8 1/8 oz mark would be. On a 2 cup measure the gap doubles to 6.8 mL, and a recipe scaled four times can drift by a tablespoon before anything reaches the oven.

Most American recipes were written against this cup whether or not the writer knew it. Butter sticks, pint jars and 8 oz glasses all rest on the same definition, which is why a European reader translating an old American recipe runs into trouble at every line.

Where 250 milliliters comes from

The metric cup is a flat 250 mL, a quarter of a liter, and it is standard in Australian, New Zealand and Canadian kitchens. An Australian tablespoon is 20 mL instead of 15 mL, so a Sydney recipe read with US spoons comes out both larger and saltier than the writer intended.

Japan uses a 200 mL cup, which explains why the scoop tucked inside a Japanese rice cooker is smaller than a US half cup measure and why the cooker's water lines do not match an American recipe. Britain ran on a 284 mL cup before metrication, half an imperial pint, and reprinted wartime cookbooks still use it.

Five standards, five answers between 200 and 284 mL. When a recipe arrives with no source, read the other units for a clue: ounces and pounds point to the US cup, grams and liters to the metric one, and a rice cooker scoop to Japan.

One cup of water on a scale

Water is the one liquid where volume and weight almost agree, which makes it the easiest thing in the kitchen to check on a scale. The USGS puts density at 1.00000 g/mL at 4 C, where water is heaviest, and 0.99802 g/mL at 21 C (Water density, USGS Water Science School). Weight is volume times density and nothing else.

At 20 C a customary cup is 236.588 times 0.9982, which is 236.2 g. The label cup weighs 239.6 g and the metric cup 249.6 g. USDA's record for tap water gives a 237 g portion for 8 fl oz (FoodData Central 173647), the same figure rounded to a whole gram.

Boiling water is 4 percent lighter than cold, 226.8 g against 236.6 g. That matters when a cup of hot water goes into a batter by weight, and not at all when the pot is filling for pasta.

Water beside milk, oil and honey in the same cup

A cup measures volume, so every liquid in the kitchen takes up the same space and almost none of them weigh the same. Water is the reference at 1.00 g/mL. Whole milk runs 1.017, because fat and protein are denser than water. Vegetable oil sits near 0.92 and floats. Honey is 1.42 and sinks to the bottom of a glass of tea.

The gap is large enough to matter in baking. A cup of honey weighs 336 g, so a recipe that lists honey by volume and butter by weight is asking for a scale whether or not it says so. Salad dressing separates for the same reason: oil rides on top of vinegar at 218 g a cup against 236 g.

Milk is close enough to water that a cup of whole milk weighs 241 g, 5 g more, and skim milk about a gram heavier again. That is why a cup of milk in a 240 mL recipe can be swapped for water with no change to the volume and a small change to the texture.

Reading the line without guessing

Put the jug on the counter, not in your hand, and crouch until your eye is level with the mark. Looking down at the line from standing height adds 5 to 10 mL of apparent volume, enough to turn one cup into one cup and a half tablespoon. Read the bottom of the curve where water climbs the glass, not the top edge of it.

Glass jugs are calibrated at 20 C and hot liquid expands, so a cup of simmering stock measured in a cold jug reads slightly high. Let it settle for a minute when the difference matters, as it does in pastry cream or a marinade.

A scale sidesteps all of it. Tare the jug, pour to 236 g for a customary cup or 240 g for a label cup, and stop. A cheap 0.1 g scale is more accurate than the printed lines on most jugs sold for home use.

One check is worth doing once: fill the jug to its 4 cup line and pour into a liter jar. A good jug leaves 54 mL of headspace. If the jar overflows, the markings are off by more than 5 percent and every recipe measured with that jug has been running wet.

One cup in coffee, tea and the glass at dinner

Coffee is where one cup of water gets measured most often. At a 1:16 brew ratio, 236 g of water takes 14.8 g of ground coffee, about three level tablespoons at 5 g each. A 1:15 ratio needs 15.7 g and a 1:17 ratio 13.9 g, which is the whole reason coffee scales exist.

Tea is lighter on the leaf: 2 to 3 g in the same cup, three minutes for black tea and two for green. A mug that holds 10 fl oz takes 296 mL, so it is 1.25 cups and wants a little more leaf than a cup measure suggests. If the kettle has already boiled, wait half a minute before pouring green tea, since water straight off the boil scorches the leaf and pulls the tannins forward.

At the table, a 250 mL tumbler is one metric cup and an old fashioned 6 fl oz juice glass is three quarters of a US cup. Eight glasses a day works out to 1.92 L in label cups and 64 fl oz in customary ones, a couple of mouthfuls apart depending on whose glass you count.

None of this changes what a glass of water does for thirst. It changes how faithfully a recipe or a fluid target reproduces someone else's numbers, which is the only thing a cup was ever meant to do.

One cup against a can, a bottle and a kettle

Water arrives in packages and vessels with their own numbers printed on them, and turning those back to cups is the fastest check on how much you are drinking.

A gallon jug pours 16 cups, so one cup is a sixteenth of the container, which is easier to picture on a jug than on a label. The quart jar is the tidy one: four cups exactly, which is why it is the standard vessel for stock or a jar of iced tea.

Cost is the other comparison. At $1.29 for a 16.9 fl oz single serving, the water in one cup costs about 61 cents. The same cup from a tap costs a fraction of a cent, and the package is the only real difference.

For a kettle, one cup heated from 20 C to 100 C soaks up 79 kJ, about 0.022 kWh. A 1,500 W kettle loses some of that to the room, so expect close to a minute, which is why a single cup is the wrong size for a pot of tea for four.