How much does a cup of flour weigh?

A cup of all-purpose flour, spooned into the cup and leveled off with a knife, weighs about 125 grams. Scoop the same cup straight into the bag and press it down as it fills, and it comes out closer to 150 grams — a fifth more flour from the same bag, in the same cup, with nothing on a scale to catch the difference.

Spooning flour into a cup and scooping the cup through the bag are two different actions, not two ways of measuring the same thing. A spoon drops flour in loosely, leaving air between the grains; pushing the cup into the bag packs it down and often mounds flour above the rim before it gets leveled off. Recipes that give cup measurements are almost always tested with the spooned-and-leveled method, so a scooped cup quietly adds a fifth more flour than the recipe assumed — enough to make a cake dense or a bread dough dry without the baker doing anything a recipe told them not to do. Sifting first goes the other direction, working air into the flour before it ever reaches the cup, which is why a sifted cup weighs less still.