There is no cups-to-grams factor

A US cup is 236.6 ml of volume, so its weight depends entirely on what is in it. Fill it with plain flour and you have about 125 g; fill it with honey and you have nearer 340 g. Nothing about the cup changed — only the density of what went in it. That is the whole reason no single conversion factor exists, and why every honest chart is a list of ingredients rather than a number you can memorise.

Worse, the flour figure assumes a particular way of filling the cup, and recipes almost never say which.

Why does a cup of flour vary so much?

Because flour compacts. The 125 g figure assumes a specific technique, and most people do not use it. Flour is meant to be spooned loosely in and struck flat with a knife. Scooping the cup through the bag compresses it on the way, and the same vessel then holds 150 g or more — a twenty per cent error introduced by the way you held your hand, before the recipe has begun.

That is a 20 per cent difference in the main structural ingredient of a cake, arriving from nothing but technique. It is the single largest source of variation in baking from American recipes, and it is invisible to anyone following the instructions exactly as written.

Which tablespoon is it?

Fifteen millilitres almost everywhere, and twenty in Australia. Three teaspoons make a tablespoon and sixteen tablespoons make a US cup, so the internal arithmetic is consistent; until the tablespoon itself changes size.

The Australian tablespoon is the trap. At 20 ml it is a third larger than everyone else’s, so an Australian recipe asking for two tablespoons of a strong ingredient. Baking soda, salt, vanilla; is asking for noticeably more than a British or American cook would give it.

The metric cup is the other mismatch: 250 ml in Australia and parts of Europe against 236.6 ml in the United States. Close enough to substitute in a stew and not in a cake.

How do you scale a recipe?

Multiply everything by the target servings divided by the original. Going from 4 to 6 is a factor of 1.5, so 250 g becomes 375 g, and then the method needs judgement rather than arithmetic.

Three things do not scale linearly.

• Pan size. This is the one that ruins the cake. Double the batter into the tin you already own and the depth doubles, so the outside is set and browning long before the centre has caught up. Two tins, or one appreciably wider one, is nearly always the right answer.
• Cooking time. A larger volume takes longer and not proportionally. Check early and use a probe or a skewer rather than the clock.
• Salt and strong flavours. These often want slightly less than the factor suggests, particularly when scaling up substantially.

Halving has its own arithmetic, which the recipe-halving article covers. The short version being to move down a unit rather than looking for a fraction that does not exist on a measuring cup.

What about calories per serving?

Divide the whole dish by the number of servings and you have a per-portion figure — 400 each from a 2,400 kcal dish split six ways. The more useful second step is to divide by the cooked weight instead: 2,400 kcal spread over 1,800 g is 133 kcal per 100 g, and that figure survives someone serving themselves a larger plate, which the per-portion one does not.

The per-100-gram figure is the one worth keeping, because it survives a disagreement about serving size. "One serving" is a decision rather than a measurement; a dish that serves six generously serves four honestly, and weighing the cooked total once gives a figure that works for any portion afterwards.

It also handles the case where the dish is eaten over several days at different portion sizes, which is the normal case rather than the exception.

Should you just weigh everything?

For anything dry, yes, and it removes this entire class of problem. A digital scale costs very little, weighing is faster than washing three cup measures, and it makes a recipe reproducible in a way volume never does.

Liquids are the exception where volume holds up well: a millilitre is a millilitre regardless of technique, and for water one millilitre is one gram anyway. The rule of thumb is to weigh solids and measure liquids, and to weigh liquids too when the recipe is a ratio rather than a list.

Questions people ask

Is a US cup the same as a metric cup? No; 236.6 ml against 250 ml, a difference of about 5.7 per cent. It matters in baking and rarely elsewhere.

Why do British recipes use grams? Because British recipe writing standardised on weight for dry ingredients, which is more reproducible. It is why a British recipe converted to cups is usually less reliable than the original.

Does sifting change the weight? Not the weight, only the volume. A cup of sifted flour weighs less than a cup of unsifted flour, which is exactly the problem weighing removes.

How do I scale a recipe by a third? Multiply by 0.667 and round sensibly. Where an ingredient is an egg or another whole unit, scale by two thirds or one and a half instead so the whole numbers work out.

Weigh the dry, measure the wet, and check which tablespoon a recipe means. The cups to grams converter carries a weight per ingredient, the cooking conversion chart covers spoons and cups across the three systems, the ingredient scale calculator applies a factor to a whole recipe, and the calories per serving calculator gives both the per-portion and the per-100-gram figure.