Understanding recipe units
A cup of flour is not a cup of butter.
Why volume-to-mass conversion needs the ingredient as an input, and the reason serious recipes weigh everything anyway.
Two kinds of conversion.
Volume-to-volume conversions are pure arithmetic — 1 cup is 236.6 ml (US) or 250 ml (metric), 1 tablespoon is three teaspoons, 1 ounce of liquid is about 29.6 ml. Those relationships hold regardless of what's inside the container. Volume-to-mass conversions are different: they depend on the density of the specific ingredient. A cup of flour weighs about 120 g; a cup of butter weighs about 227 g; a cup of honey weighs about 340 g. The "right" conversion factor isn't a constant.
mass = volume × density(ingredient)
Why volume is unreliable for dry ingredients.
A cup of flour can weigh anywhere from 100 g (sifted, scooped lightly) to 145 g (dipped and packed) — a 45 % range from the same word "cup". The variability is the reason most professional bakers refuse to use cups at all. Bread, pastry and laminated doughs are sensitive to hydration ratios within a few percent; if "1 cup of flour" can vary by a third, the recipe's water-to-flour balance is shot before you've started. The definition of a cup is fine; what's broken is asking the cook to compress an uncompressible quantity into a fixed shape consistently.
The US-vs-metric cup mismatch.
The US cup is 236.6 ml (16 US tablespoons of ½ fluid ounce each). The metric cup (Australia, New Zealand, sometimes Canada) is exactly 250 ml. The UK never uses cups for cooking — it uses fluid ounces (28.4 ml) and pounds. A recipe converted from US to metric volumes by the wrong cup is off by 5–6 %, which is small enough to ignore for stew and large enough to ruin macarons. When converting, always identify the cup standard first.
A worked example.
"2½ cups of all-purpose flour" — US cups by convention if the recipe is American. 2.5 × 236.6 ml = 591.5 ml of volume. Flour density ≈ 0.51 g/ml when scooped normally, so mass ≈ 591.5 × 0.51 ≈ 302 g. Confirm against the standard 120 g per cup figure: 2.5 × 120 = 300 g. The two routes agree within rounding because both use the same underlying density. Now do the same with butter — 0.96 g/ml — and you'd get 568 g, not flour's 302 g. Same volume, double the mass.
2½ cups all-purpose flour
1 US cup = 236.6 ml ; flour density ≈ 0.51 g/ml
Volume × density gives mass. Always specify ingredient.
2.5 × 236.6 × 0.51 ≈ 301.7
= ≈ 302 g
2½ cups butter (same volume!)
Butter density ≈ 0.96 g/ml
The volume is identical to the flour case; the mass isn't.
2.5 × 236.6 × 0.96 ≈ 567.8
= ≈ 568 g
Ingredient densities to know.
A handful of densities cover most recipes: water 1.00 g/ml; milk 1.03; cream 0.99; honey 1.42; maple syrup 1.32; granulated sugar 0.85; brown sugar (packed) 0.93; all- purpose flour 0.52; cake flour 0.42; whole-wheat flour 0.55; cocoa powder 0.50; butter 0.96; vegetable oil 0.92; salt (fine) 1.20; salt (kosher, coarse) 0.66. Notice salts differ by a factor of nearly two — kosher salt vs table salt is one of the most common-failure conversions in recipes that travel.
Spoons are surprisingly consistent — for liquids.
A tablespoon is 15 ml almost everywhere except Australia (20 ml) and a few specific contexts in the US (14.79 ml). A teaspoon is 5 ml almost everywhere. For liquid measurements, "1 tbsp" is one of the more reliable units a recipe can specify. For dry ingredients, the same compression issue applies as with cups — "1 tbsp of flour" can vary from 7 g to 10 g depending on whether the cook leveled it. Salt and spice measurements are the only place where this variance reliably doesn't matter.
When precision matters.
For stews, casseroles, soups and most savoury cooking, volume measurements are fine — the technique tolerates wide ratios. For yeasted bread, laminated pastry, macarons, sponge cakes, sourdough, anything where hydration percentages matter — switch to grams and weigh everything. A £15 kitchen scale removes 90 % of recipe-failure variance in baking. The converter here helps you cross between the two worlds; the recommendation is to live mostly in grams once you start caring about consistency.