Why a cup of one thing is not a cup of another
Volume measures how much space something occupies; mass measures how much of it there is. The bridge between them is density, and for baking ingredients the relevant figure is bulk density — the weight of the material plus the air trapped between its particles as it sits in the cup. Water has no such air and comes in at 236.6 g per US cup, exactly its volume in millilitres since water is 1.000 g/mL. Flour, which is a fine powder full of voids, weighs about half that. Honey, which is denser than water and has no voids at all, weighs 336 g.
That spread is why cup measurement is unreliable for baking in a way it is not for cooking. A stew does not care whether the onion was 180 g or 220 g. A cake batter does, because the ratio of flour to liquid to fat determines the crumb, and an error in one of them is not corrected by the others. Professional and metric recipes are written in grams for this reason, and every serious baking reference published in the last two decades supplies a weight chart alongside its cup measures.
The second reason is repeatability. A cup of flour dipped straight from the bag is compacted by the scoop and weighs materially more than a cup spooned in loosely and levelled with a knife. The same cook, the same cup and the same flour can produce a range of weights depending only on technique. A scale removes the variable entirely.
The calculation and where the densities come from
The arithmetic is one multiplication once the volume is in cups: mass equals volume times density times any scale factor. Getting the volume into cups is the only preliminary step, and it uses the exact US measures — a tablespoon is one sixteenth of a cup, a teaspoon one forty-eighth, and a cup is 236.5882365 mL. A metric cup is 250 mL, so a metric cup of anything is 5.7% heavier than a US cup of the same thing.
The densities themselves come from two kinds of source, and it is worth knowing which is which. For liquids and fats, the figure is derived from a physical density: water at 1.000 g/mL gives 236.6 g per cup, vegetable oil at about 0.92 g/mL gives 218 g, and honey at about 1.42 g/mL gives 336 g. Butter is a special case where the US convention is exact by packaging rather than by density — a cup is two sticks, which is half a pound, or 227 g.
For dry goods, the figure is a measured bulk density published by a baking authority, and different authorities differ. All-purpose flour is given as 120 g per cup by King Arthur Baking, whose chart assumes flour spooned into the cup and levelled; other widely used charts give 125 g or more, generally because they assume a different filling method. That is a genuine 4% spread and it is not a rounding error — it reflects a real difference in how much flour ends up in the cup. This calculator uses the spooned-and-levelled convention, and the custom field lets you enter whatever figure your own recipe supplies.
The practical implication is that you should follow one chart consistently rather than mixing them. A recipe whose author weighed a cup of flour at 125 g and whose ratios were tuned around that will behave slightly differently if you weigh 120 g. It will still work — a 4% flour difference is inside normal variation — but consistency is what makes a recipe reproducible.
Worked example: converting a cup-based cake recipe to weights
A recipe calls for 2 cups all-purpose flour, 1½ cups granulated sugar, ½ cup butter and ¾ cup whole milk, and you want to weigh everything and make one and a half times the quantity.
- Flour. 2 cups × 120 g = 240 g, then × 1.5 = 360 g.
- Sugar. 1.5 cups × 198 g = 297 g, then × 1.5 = 445.5 g.
- Butter. 0.5 cup × 227 g = 113.5 g, then × 1.5 = 170.25 g. In US terms that is one and a half sticks, since a stick is 113.4 g.
- Milk. 0.75 cup × 244 g = 183 g, then × 1.5 = 274.5 g. Milk is close enough to water that weighing and measuring agree within a few per cent, but weighing is still faster.
- Check the ratios. In the scaled recipe, sugar is 445.5 ÷ 360 = 1.238 times the flour weight, exactly as it was before scaling (297 ÷ 240 = 1.238). Scaling multiplies every mass by the same factor, so every ratio survives.
- What does not scale. The pan. One and a half times the batter needs about one and a half times the pan area at the same depth, and the bake time changes with depth rather than with volume. The baking pan size conversion calculator works that out.
How much the density figure matters
Sort your ingredients by how much a density error costs. For structural ingredients — flour, liquid, eggs, fat — a 5% error shifts the recipe's ratios and shows up in the result. For flavouring ingredients — vanilla, spices, zest — a 20% error is invisible. For chemical leavening, precision matters more than for anything else, because baking powder and soda act in a narrow band; but they are measured in teaspoons, and a level teaspoon of a fine powder weighs only a few grams, so weighing them on a domestic scale that resolves to 1 g is not obviously better than measuring them carefully by spoon.
Where a recipe already gives weights, use those and ignore the cup figure entirely. Where it gives only cups and does not say which convention it used, the flour figure is the one worth thinking about: if your bakes come out consistently dry, you may be weighing more flour than the author measured, and vice versa. Adjusting the flour weight by a few per cent between bakes is a legitimate way to calibrate a recipe to your own chart.
Finally, note that the density here is for the ingredient in the state named. Brown sugar is quoted packed, which is the standard convention and gives 213 g per cup; loosely spooned brown sugar is considerably lighter. Confectioners' sugar is quoted unsifted; sifting adds air and lowers the weight per cup. Sifted, packed and levelled are not adjectives you can skip.
Common baking ingredients by weight per cup
| Ingredient | Grams per cup | Grams per tablespoon | Ounces per cup | Density (g/mL) |
|---|---|---|---|---|
| Cocoa powder, unsweetened | 85 | 5.31 | 3.00 | 0.359 |
| Rolled oats | 89 | 5.56 | 3.14 | 0.376 |
| Cake flour | 113 | 7.06 | 3.99 | 0.478 |
| Whole wheat flour | 113 | 7.06 | 3.99 | 0.478 |
| Confectioners' sugar, unsifted | 113 | 7.06 | 3.99 | 0.478 |
| All-purpose flour, spooned | 120 | 7.50 | 4.23 | 0.507 |
| Bread flour | 120 | 7.50 | 4.23 | 0.507 |
| Chocolate chips | 170 | 10.63 | 6.00 | 0.719 |
| Granulated sugar | 198 | 12.38 | 6.98 | 0.837 |
| Vegetable oil | 217.7 | 13.61 | 7.68 | 0.920 |
| Brown sugar, packed | 213 | 13.31 | 7.51 | 0.900 |
| Butter | 227 | 14.19 | 8.01 | 0.959 |
| Water | 236.6 | 14.79 | 8.35 | 1.000 |
| Whole milk | 244 | 15.25 | 8.61 | 1.031 |
| Honey | 336 | 21.00 | 11.85 | 1.420 |
Every tablespoon column is the cup figure divided by 16, and every density column is the cup figure divided by 236.5882365.
What goes wrong when converting cups to grams
- Using one density for everything. The range in the table above is nearly fourfold, from 85 g for cocoa to 336 g for honey. A single conversion factor is wrong for almost every ingredient.
- Ignoring the filling method. Flour dipped from the bag is compacted and weighs more per cup than flour spooned in and levelled. The figures here assume spooned and levelled; if your recipe assumed dipping, its cup was heavier.
- Mixing charts between ingredients. Take all your weights from one source. Combining a 125 g flour cup from one chart with a 198 g sugar cup from another distorts the ratio between them even if each figure is defensible on its own.
- Forgetting that a metric cup is bigger. A 250 mL metric cup holds 5.7% more than a 236.59 mL US cup, so a metric cup of flour is about 127 g at the same density.
- Assuming the scale factor applies to everything. Ingredient masses scale linearly; pan area, bake time, and salt in a long fermentation do not. Scale the ingredients here, then reconsider the process.
- Weighing tiny quantities on a coarse scale. A scale that resolves to 1 g cannot usefully weigh 4 g of baking powder. Measure small quantities by spoon and reserve the scale for anything above about 20 g.
Related baking and measurement tools
This page converts volume to mass for one ingredient at a time. If what you actually need is to move a recipe between measuring systems — US cups to metric cups, or tablespoons to millilitres — the cooking measurement conversion calculator handles the volume side across four national conventions without involving density at all. For the reverse of this page, starting from a weight and asking what volume it occupies, use the ingredient weight to volume calculator.
Once everything is in grams, bread and pastry are best handled as ratios rather than absolute quantities. The baker's percentage calculator expresses every ingredient as a percentage of flour weight, which makes a formula scale to any batch size and makes two recipes directly comparable. The dough hydration calculator applies the same idea to the water-to-flour ratio specifically.
If you are resizing rather than converting, the recipe scaling calculator multiplies the whole ingredient list at once and is the better tool when a recipe has a dozen lines. And for the weight units themselves — grams against ounces and pounds — the ounces to grams calculator covers the conversion exactly.