A cup of flour is not a fixed amount
Scooped or spooned, the same cup holds 25 per cent more or less flour. That range is the difference between tender and dry.
The 25 per cent problem
Dip a cup into a bag of flour and it compacts: about 150 g. Spoon flour in loosely and level it: about 120 g. Same cup, same flour, a quarter more in one than the other — and no way to tell by looking.
Which ingredients are stable
| Ingredient | Per US cup | Reliable? |
|---|---|---|
| Water | 237 g | Yes |
| Granulated sugar | 200 g | Mostly |
| Plain flour | 120–150 g | No |
| Brown sugar | 145–220 g | No |
| Cocoa powder | 85–100 g | No |
Anything that compacts or clumps is unreliable by volume. That covers most of what makes a cake work.
Two cups, two sizes
A US cup is 236.6 ml. A metric cup is 250 ml. Six per cent does little to a glass of water and quite a lot to a batch of scones.
The tin is a conversion too
American recipes give tins in inches. A 9-inch round is 22.86 cm and holds roughly 30 per cent more than a 20 cm tin, so the same mixture bakes shallower and cooks faster. Start checking ten minutes before the stated time whenever you change tin size.
The fix costs almost nothing
A scale reading to one gram removes every ambiguity above. Weigh into the bowl, note the grams once, and the recipe becomes repeatable rather than lucky.
Why density varies so much
Flour is a powder with air between the particles, and the proportion of air depends entirely on handling. Scooping compresses it; sifting aerates it; a bag that has travelled on a pallet is denser at the bottom than the top. None of this is visible in a measuring cup.
The effect on a bake is direct. Twenty-five per cent more flour in a cake batter means a stiffer crumb, less rise and a drier result. Bakers who insist their recipe "works better in grams" are not being fussy; they are removing a genuine variable.
A conversion table you can trust
| Ingredient | 1 US cup | 1 tbsp |
|---|---|---|
| Plain flour (spooned) | 125 g | 8 g |
| Granulated sugar | 200 g | 12.5 g |
| Icing sugar | 120 g | 7.5 g |
| Butter | 227 g | 14 g |
| Cocoa powder | 85 g | 5 g |
| Rolled oats | 90 g | 5.5 g |
| Water or milk | 237 g | 15 g |
Treat these as the writer's intent when a recipe gives cups without weights — they match the spooned-and-levelled convention used in most American cookbooks.
Tin sizes and why area matters
Batter depth drives baking time far more than tin diameter does, and depth depends on area. A 20 cm round tin has an area of 314 cm²; a 23 cm tin has 415 cm² — 32 per cent more. Pour one recipe into the larger tin and the batter sits a third shallower, browning faster and drying at the edges.
| Imperial | Metric | Area |
|---|---|---|
| 7 in round | 17.8 cm | 249 cm² |
| 8 in round | 20.3 cm | 324 cm² |
| 9 in round | 22.9 cm | 412 cm² |
| 10 in round | 25.4 cm | 507 cm² |
Adjusting when the tin is wrong
Scale the recipe by the ratio of areas rather than diameters. Moving from a 9-inch to an 8-inch tin means multiplying every ingredient by 324 ÷ 412, or about 0.79. Keep the temperature the same and expect a slightly longer bake, since the batter is deeper.
Oven temperature, honestly
350°F is 176.7°C, and every European recipe rounds it to 180°C. Domestic ovens routinely run 10 to 15 degrees away from their dial setting, so precision beyond 5°C is theatre. An oven thermometer tells you more than any conversion table.
The takeaway
Weigh dry ingredients, convert tin sizes by area rather than diameter, and stop treating cups as a unit of mass.
Filed under Baking · Tags: cups to grams, baking, kitchen scales. Figures verified against the official definitions — see our editorial policy.