Four cups is 32 fluid ounces, and that part never changes

Four US customary cups hold 32 US fluid ounces, which is 946.352946 milliliters. The chain is short: a cup is 8 fluid ounces by definition, a fluid ounce is 29.5735295625 mL, so 4 x 8 x 29.5735295625 = 946.352946 mL. No ingredient enters that arithmetic, because cups and fluid ounces both describe space.

The same four cups is 64 tablespoons, 192 teaspoons, 1 quart, 2 pints and a quarter of a US gallon. Those are all exact fractions of the cup, so they hold for any liquid you can pour, from stock to paint.

Worked example, in full: 4 cups x 8 fl oz per cup = 32 fl oz. Then 32 fl oz x 29.5735295625 mL per fl oz = 946.352946 mL. Going back, 946.352946 / 236.5882365 = 4 cups, which is the check that the two constants are consistent.

A fluid ounce measures space, an ounce measures mass

This is the trap in the question. The fluid ounce is a volume of 29.5735295625 mL. The ounce is a mass of 28.349523125 g. They share a name and nothing else, and the coincidence that a fluid ounce of water weighs close to an ounce is what keeps the confusion alive.

Put numbers on it. Four cups of water is 946.352946 mL, and water at 4 C has a density of 1.000 g/mL, so the water weighs 946.352946 g. Divide by 28.349523125 and that is 33.3816 oz on a scale, against the 32 fl oz of volume. The 1.38 oz gap is 4.3 percent, and it exists for water, the densest common liquid at that temperature.

Choose a denser liquid and the gap widens. Four cups of honey is still 32 fl oz of volume, but the scale reads 47.83 oz, half again as heavy as the same volume of water. Choose flour and the scale reads 17.64 oz, roughly half the water figure. The volume answer of 32 fl oz is right in all three cases, and it is the only figure that stays put.

A recipe that says "add 32 ounces of stock" is ambiguous for this reason. If the writer meant fluid ounces, the measure is a quart. If the writer meant weight ounces, the measure is 907 g of stock, which at a density near 1.01 g/mL is about 3.8 cups. Professional recipes settle the question by writing fl oz or oz, and by weighing dry ingredients.

Four cups weighs 17.6 oz of flour or 47.8 oz of honey

Weight per cup is a property of the food, not of the cup. USDA FoodData Central publishes a portion weight for a one cup measure on many foods, and the numbers spread widely: all-purpose flour at 125 g per cup (FDC 168936), granulated sugar at 200 g per cup (FDC 169655), honey at 339 g per cup (FDC 169640).

Two of those rows worked out in full. Flour: 4 cups x 125 g = 500 g, then 500 / 28.349523125 = 17.636981 oz. Honey: 4 x 339 = 1,356 g, then 1,356 / 28.349523125 = 47.831492 oz. The ratio between them is 339 / 125 = 2.712, so the honey is 2.7 times the weight of the flour in the same four cups.

The widget on this page runs that table for any cup count you type, for all five ingredients, and reports fluid ounces, milliliters, tablespoons, teaspoons, grams, weight ounces, pounds and quarts at once. The cup figure and the fluid ounce figure drive each other, so you can start from either side.

How the flour is scooped moves four cups by 100 g

Flour is the ingredient where cup measures earn their bad reputation. A cup spooned into the measure and leveled weighs about 125 g, the figure USDA publishes. The same flour scooped straight from the bag with the cup compacts under the rim and can weigh 20 percent more, which takes four cups from 500 g to roughly 600 g, or from 17.64 oz to 21.16 oz.

Published charts differ in the same direction. The King Arthur Baking ingredient weight chart lists 120 g for a cup of all-purpose flour, so four cups is 480 g there, while USDA says 500 g. The 20 g spread is 4 percent, and it is why two bakers following the same recipe in cups can end up with doughs of different hydration.

Sugar behaves better because the crystals do not compact much: 200 g per cup is stable whether you spoon or scoop, so four cups is 800 g within a few grams. Honey is the opposite problem, since it clings to the walls of the cup, and the volume you poured and the volume you measured part company. Weighing the jar before and after avoids that loss, and the gap grows when you scale a recipe by four.

Scaling four cups to quarts, pints and thirds

Four cups is exactly one quart, and that is the most useful equivalence in US cooking because stock, cream and canned tomatoes are sold by the quart. Two pints is the same volume, and 64 tablespoons or 192 teaspoons gets you to the same place with spoons.

A third of four cups is a common ask when a recipe serves three instead of four. The arithmetic is 4 / 3 = 1.3333 cups, which is 10.667 fl oz and 315.451 mL, from 946.352946 / 3 = 315.450982 mL. Halving is easier: 2 cups, 16 fl oz, 473.176473 mL. Doubling gives 8 cups, 64 fl oz and 1,892.705892 mL, which is half a gallon.

Weight does not scale the same way when you change the vessel. Doubling four cups of flour to eight cups does not double the weight if the second four cups are packed tighter, which is the argument for converting the recipe to grams once and multiplying the gram figure instead.

Dry cups and liquid cups hold the same 236.6 mL

The two kinds of measuring cup in a US kitchen are not different sizes. A liquid cup and a dry cup both hold 236.5882365 mL, the US customary cup. What differs is the shape: the liquid cup has a spout and a scale you read at eye level at the meniscus, while the dry cup is filled to the rim and leveled with a straight edge.

Because the capacity matches, a recipe that calls for four cups of milk can be measured in either cup without changing the volume. The technique still matters, since a liquid cup read from above looks fuller than it is, and a dry cup of flour shaken down holds more than one spooned in.

Outside the US the cup changes. The metric cup used in Australia, Canada and New Zealand is 250 mL, so four metric cups is 1,000 mL against 946.35 mL for four US cups, a difference of 5.67 percent, from 250 / 236.5882365 = 1.05669. A British recipe from before metrication used the imperial cup of 284.13 mL, and four of those is 1,136.52 mL, a fifth more than the US figure.

US labels follow the federal rules in 21 CFR 101.9, which allow household measures such as cups and fluid ounces next to the metric declaration, so a bottle of juice may state both 8 fl oz and 240 mL for the same serving.

When four cups should go on a scale instead

Volume measures carry more uncertainty than a scale for anything dry. A cup of flour measured by hand varies by 10 to 20 g, which on four cups is 40 to 80 g, or 3 to 6 percent of 500 g. A kitchen scale reading to 1 g on the same 500 g carries 0.2 percent. The scale wins by an order of magnitude.

Packaged food sold by volume has a parallel rule set. NIST Handbook 133 covers how officials check a declared net quantity, whether the label states fluid ounces or weight ounces, and the check accounts for the density of the product when a volume declaration is verified by weighing. That is the same density table a cook uses, applied for a different reason.

The practical routine for four cups of anything dry: put the bowl on the scale, tare it, and add the gram figure from the table above. For four cups of all-purpose flour that is 500 g at the USDA portion weight, or 480 g on the King Arthur chart. Note which one the recipe author used, and the result will repeat next time.