Why a cup of one thing is not a cup of another
A US customary cup is a fixed volume: 236.588 mL, or exactly 8 US fluid ounces. What it weighs depends entirely on what you put in it. A cup of cocoa powder weighs 85 g. A cup of honey weighs 340 g — four times as much, in the same container.
The bridge between the two is bulk density: mass per unit volume, including the air between the particles. Honey has essentially no trapped air and a density around 1.44 g/mL. Cocoa powder is mostly air by volume and comes in near 0.36 g/mL. Flour sits at about 0.51 g/mL when you spoon it into the cup, and rather more when you scoop the cup through the bag and compress it on the way.
That last point is the reason professional bakers weigh. Volume measurement of a compressible powder is not repeatable, because the measurement changes what you are measuring. Two cooks working from the same recipe, the same bag and the same cup can differ by 20 g of flour, which on a 250 g cake formula is an 8% difference in the single ingredient that controls structure. Weight removes the variable entirely: 120 g is 120 g whoever scoops it.
This calculator does not use one blanket conversion. It carries a separate reference weight for each ingredient — and for flour, a separate one for each measuring method — then converts in whichever direction you need.
The conversion, and where the reference numbers come from
The arithmetic is one multiplication. Density ρ is the reference weight of one cup divided by 236.588 mL. Mass is then volume × density; volume is mass ÷ density. Everything else on the page — tablespoons, fluid ounces, ounces avoirdupois — is a unit change on one side or the other.
The reference weights come from three different kinds of source, and it is worth knowing which is which.
Liquids are computed from physical density. Water is 1.000 g/mL by the original definition of the gram, so a cup is 236.6 g. Whole milk is a shade denser at about 1.03 g/mL because of its dissolved lactose and protein, giving 244 g. Oil is lighter at about 0.92 g/mL, so a cup is 218 g. Beware of charts that list liquids at 227 g per cup: that is 8 avoirdupois ounces, and the fact that a US cup is also 8 fluid ounces is a coincidence of naming that only holds true for water-like liquids by accident.
Dry baking goods use published test-kitchen weights, principally the King Arthur Baking ingredient weight chart: 120 g for a cup of spooned all-purpose flour, 198 g for granulated sugar, 213 g for packed brown sugar, 85 g for cocoa. The dipped-flour figure of 142 g per cup is the 5-ounce cup that America's Test Kitchen standardised on, and it is included precisely so you can see the size of the gap.
Salts use the manufacturers' own nutrition panels, which is the only reliable source for them, because the weight per spoon depends on the crystal shape that brand produces. Butter is a special case: one cup is 227 g because a stick is legally a quarter pound and is wrapped as half a cup, not because anyone measured it. That convention implies a density of about 0.96 g/mL, slightly above butter's real value of just over 0.9 — a definition, not a measurement, but the one every American recipe assumes.
Worked example: a cup-based cake recipe converted to grams
You want to weigh a recipe calling for 2 cups all-purpose flour, 1½ cups granulated sugar, 1 cup whole milk, ½ cup butter and 1 teaspoon table salt. Convert each line.
- Flour. The recipe is American and does not say how to fill the cup, so assume spooned: 120 g per cup. 2 × 120 = 240 g. Had the author dipped, it would be 2 × 142 = 284 g — 44 g more, which is why so many home bakes come out dry.
- Sugar. 198 g per cup. 1.5 × 198 = 297 g. Granulated sugar does not compress, so this one is genuinely reliable by volume.
- Milk. Density 1.031 g/mL × 236.588 mL = 244 g.
- Butter. 226.8 g per cup, so half a cup is 113 g — one stick, which is exactly how the recipe should have written it.
- Salt. Table salt is 288 g per cup and there are 48 teaspoons in a cup, so 288 ÷ 48 = 6.0 g. If you use Diamond Crystal kosher instead, the same teaspoon is 134 ÷ 48 = 2.8 g, less than half. Substituting one for the other by the spoonful is how a dish ends up inedible.
Total batter weight before eggs and leavening: 240 + 297 + 244 + 113 + 6 = 900 g. Recording that total is what makes the recipe portable — you can now halve it, scale it in the recipe scaling calculator, or match it to a pan with the baking pan size conversion calculator without ever touching a measuring cup again.
How to check whether a conversion is plausible
Use the density output as your sanity check. Almost every kitchen ingredient falls between 0.3 and 1.5 g/mL, and knowing roughly where yours should sit catches typing errors instantly.
Anything that pours and is mostly water — milk, cream, stock, juice, wine — sits within a few percent of 1.0 g/mL. Fats and oils sit just below 1.0, around 0.9. Sugar syrups and honey sit well above, from 1.3 upward, because sugar dissolved in water is denser than either. Dry powders that trap air — flour, cocoa, confectioners' sugar, oats — land between 0.35 and 0.6. Crystalline solids that pack without air pockets, like table salt and granulated sugar, come in around 0.8 to 1.2.
If a conversion gives you a density outside those bands, you have either chosen the wrong ingredient or entered the wrong unit. The classic error is entering a fluid-ounce figure in the ounces field, or the reverse: a US fluid ounce is 29.57 mL of volume, an avoirdupois ounce is 28.35 g of mass, and they are not interchangeable for anything but water.
The other check is the total. Weigh your finished mixing bowl and compare it against the sum of the converted lines. If they disagree by more than a couple of percent, one of your conversions is wrong — and it is almost always the flour.
Reference weights per US cup
| Ingredient | g per cup | g per tbsp | Density (g/mL) |
|---|---|---|---|
| All-purpose flour, spooned & levelled | 120 | 7.5 | 0.507 |
| All-purpose flour, dipped & swept | 142 | 8.9 | 0.600 |
| Bread flour | 120 | 7.5 | 0.507 |
| Cake flour | 120 | 7.5 | 0.507 |
| Whole wheat flour | 113 | 7.1 | 0.478 |
| Granulated sugar | 198 | 12.4 | 0.837 |
| Brown sugar, packed | 213 | 13.3 | 0.900 |
| Confectioners' sugar, unsifted | 113 | 7.1 | 0.478 |
| Butter | 227 | 14.2 | 0.959 |
| Water | 236.6 | 14.8 | 1.000 |
| Whole milk | 244 | 15.3 | 1.031 |
| Heavy cream | 235 | 14.7 | 0.993 |
| Vegetable oil | 218 | 13.6 | 0.921 |
| Honey | 340 | 21.3 | 1.437 |
| Cocoa powder, unsweetened | 85 | 5.3 | 0.359 |
| Rolled oats | 89 | 5.6 | 0.376 |
| Chocolate chips | 170 | 10.6 | 0.719 |
| Table salt, fine | 288 | 18.0 | 1.217 |
| Diamond Crystal kosher salt | 134 | 8.4 | 0.566 |
| Morton kosher salt | 230 | 14.4 | 0.972 |
Dry baking ingredients follow the King Arthur Baking ingredient weight chart; the dipped-flour figure is the 5 oz per cup convention used by America's Test Kitchen; liquids are computed from physical density; salts are derived from the brands' own nutrition panels. Densities are the cup weight ÷ 236.588 mL.
The errors that actually change a recipe
- Confusing fluid ounces with ounces. A US fluid ounce is 29.57 mL of volume; an ounce avoirdupois is 28.35 g of mass. Only water makes them nearly equal.
- Using an imperial or metric cup. The UK imperial cup is 284 mL and the Australian metric cup is 250 mL. A US cup is 236.6 mL. An Australian cup of flour is 6% heavier than an American one.
- Swapping salt brands by volume. A teaspoon of Diamond Crystal kosher weighs 2.8 g against 6.0 g for fine table salt. Convert by weight or the dish is more than twice as salty.
- Assuming sifted and unsifted are the same. "One cup sifted flour" means sift, then measure. "One cup flour, sifted" means measure, then sift. They are different weights, and the word order carries the whole meaning.
- Packing brown sugar loosely. The 213 g figure assumes it is pressed firmly into the cup, which is what every recipe intends when it writes "packed".
- Weighing butter by the wrapper markings on a partly used stick. The lines drift once the stick has softened and been re-wrapped; weigh it instead.
- Rounding too early. Convert each line at full precision, then round to whole grams once, or a long ingredient list accumulates several grams of drift.
Bulk density is not a fixed property
The reference weights here are representative, not exact constants. A cup of flour varies with humidity, how long the bag has been sitting, whether the flour was sifted, how sharply you levelled it, and how wide the cup's mouth is. Published charts from different test kitchens disagree by a few percent for exactly this reason, and none of them is wrong.
That variability is the argument for weighing rather than a flaw in the conversion. If a specific brand matters to you — a stone-ground flour, a coarse raw sugar, a particular cocoa — weigh one levelled cup of it once, note the number, and enter it under the custom option. Your own measurement beats any chart.
When to convert and when to leave it alone
Convert to weight whenever the ingredient is compressible, expensive, or structural. Flour, cocoa, ground nuts and confectioners' sugar are all worth weighing every time. So is anything you will scale, because multiplying a weight is exact and multiplying "⅔ cup plus 2 tablespoons" is not.
Leave volume alone for liquids measured in a jug, for small quantities of free-flowing ingredients where the absolute error is trivial, and for anything where the recipe's tolerance is wide — a splash of oil in a pan does not need a scale. A teaspoon of vanilla measured by spoon is fine; a teaspoon of salt is not, for the reasons above.
Weight also unlocks percentage-based formulas, which is how bakeries actually work. Once every ingredient is a number of grams, you can express the whole formula against flour weight in the baker's percentage calculator, hit a hydration target with the dough hydration calculator, or cost a recipe out per portion in the recipe cost per serving calculator. None of those are possible from a cup measure.
One last practical note. Buy a scale that reads in 1 g increments and has a tare button, and put the mixing bowl on it. You will use fewer tools, wash less, and stop converting anything at all.