UnitBench

Cups to grams, by ingredient

A cup of flour is 125 g. A cup of water is 237 g. The ingredient is not an optional detail.

Volume and weight are not interchangeable

A cup measures space. A gram measures mass. Converting between them requires knowing how heavy the thing is for its size — its density — and ingredients differ enormously:

Ingredient1 US cup1 metric cup (250 mL)
Water237 g250 g
Milk244 g258 g
Plain flour (spooned)125 g133 g
White sugar201 g213 g
Brown sugar (packed)220 g233 g
Butter216 g228 g
Honey336 g355 g
Rolled oats95 g100 g

Flour and honey differ by a factor of nearly three for the same cup. This is why a converter that answers "cups to grams" without asking what is in the cup cannot be right for more than one ingredient.

Flour is the unreliable one

Flour compacts. Scooping the cup directly into the bag packs it down and can add 20–25% over the same cup filled by spooning flour in and levelling it off. That difference is enough to turn a cake dry, and it is the single biggest reason two people following the same recipe get different results.

The figure here assumes the spoon-and-level method. If you want to remove the uncertainty entirely, weigh it — which is precisely why professional and most modern recipes are written in grams.

Cups are not standard across countries either. A US cup is 236.6 mL; an Australian or New Zealand metric cup is exactly 250 mL. Australian tablespoons are 20 mL against 15 mL elsewhere — four teaspoons rather than three, which matters wherever baking powder does.

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Questions

Why can't a cup just be converted to grams?
Because a cup is a measure of volume and a gram is a measure of weight, and the bridge between them is the density of whatever you are measuring. A US cup of water weighs about 237 g; the same cup of plain flour weighs about 125 g, and of honey about 336 g. Any converter offering a single "cups to grams" answer without asking what the ingredient is has picked one for you.
How accurate is the flour figure?
Less accurate than the others, unavoidably. Flour compacts: scooped straight from the bag it can weigh 20–25% more than the same cup spooned in gently and levelled. The figure used here assumes the spoon-and-level method, which is the standard most recipes are written against. This variability is the reason serious baking recipes give weights.
Is a cup the same everywhere?
No. A US cup is 236.6 mL, a metric cup (Australia, New Zealand) is exactly 250 mL, and a Japanese cup is 200 mL. That is a 6% difference between US and metric — enough to matter in baking and not in a stew. Both are in the unit list, labelled.
What about tablespoons?
Also inconsistent: 14.8 mL in the US, 15 mL in most metric countries, and 20 mL in Australia. The Australian tablespoon is the one that surprises people, and at four teaspoons rather than three it is a genuine trap in a recipe with any leavening in it.