One cup is not one weight

A cup measures volume; a gram measures mass. The ingredient and the way it fills the cup determine the weight. Flour contains air between particles, so scooping or pressing it can add more flour to the same space. That can change a batter even when the number of cups looks correct. Never turn millilitres of water into grams of flour by simply keeping the number.

First, identify the cup

A US customary cup is 236.5882365 mL, a metric cup is 250 mL, and the US nutrition-labeling convention is 240 mL. These are different definitions, not three precision settings for one unit. Check the recipe's measurement notes and your measuring cup. If the source does not identify its cup, ask rather than silently substituting your local standard.

Follow the recipe's own weight

If a recipe supplies grams, use those grams instead of recalculating them from a general chart. King Arthur Baking uses 120 g for a cup of its all-purpose flour. That is a useful stated convention, not a universal weight for every flour, brand or measuring method. An ingredient-specific chart is a fallback when the recipe supplies no weight; record which reference you chose.

A worked example, with the assumption visible

Suppose a recipe calls for 2.5 cups of flour and you deliberately adopt 120 g per cup. The calculation is 2.5 × 120 = 300 g. Put an empty bowl on the scale, tare it, then add 300 g of flour. Do not apply the same factor to sugar, butter or a different flour. Changing the ingredient is a separate recipe decision, not merely a unit conversion.

Before the next bake

Check the cup standard, ingredient and measuring instructions; prefer an explicit recipe weight; note any assumed grams-per-cup factor. Without a scale, loosen the flour, spoon it gently into a dry measuring cup and level the top without packing it, unless the recipe specifies another method.

If the result remains dry or dense, investigate baking time, oven temperature and mixing as well. A converter can correct arithmetic, but cannot diagnose the entire recipe or measure the flour in your cupboard.