All-purpose flour

One US cup of all-purpose flour weighs between 120 and 141.7 grams (4.2 to 5 ounces), depending on how the cup is filled and which source measured it. Use 120 grams (4.2 ounces) if you spoon the flour into the cup and level it. Every figure behind it is below, with the method each source used.

Also called plain flour, AP flour, white flour.

A cup of all-purpose flour weighs 120 g (4.2 oz) if you spoon the flour into the cup and level it, and about 142 g (5 oz) if you dip the cup into the bag and sweep it level. USDA's 125 g does not say which. How the cup is filled changes the weight by up to a fifth, more than any difference between brands, so the method matters more than the number.

All-purpose flour is milled from wheat to a middle protein level, about 10 to 12%, and is the general flour for cookies, cakes, quick breads and most yeast doughs. Like every fine flour it is cohesive: it holds air when it is loosened and compacts when it is pressed, so the same bag fills a cup lightly or densely depending on how the cup is filled.

Three sources measure it three ways. King Arthur gives 1 cup of its all-purpose flour as 4 1/4 oz, 120 g, and tells the baker to spoon the flour gently into the cup and sweep off the excess. America's Test Kitchen dips the cup into the bag and levels it, and gives 5 oz, 141.75 g, averaged from many testers; it notes that technique alone can change the weight by up to 20%. USDA's Standard Reference record (FDC 168894) gives a cup as 125 g without saying how it was filled. Each figure belongs to its own method, and no method has a second independent source yet, so the page shows all three and recommends the spooned figure rather than averaging them.

The physical check: wheat flour particles weigh about 1.45 g/mL. A spooned cup at 120 g is about 35% flour and 65% air; a dipped cup at 142 g is about 41% flour. Sifting before measuring adds still more air, and King Arthur's own measuring guide notes that a firmly compacted cup can reach 160 g. A cup of flour is mostly air, and the method decides how much.

Recipes follow their authors' method. King Arthur recipes, and most recipes that tell you to spoon and level, are written for 120 g cups; America's Test Kitchen and Cook's Illustrated recipes are written for 5 oz cups, so use their weights with their recipes. Bread flour behaves the same way: 120 g spooned by King Arthur and 137 g by USDA.

Practical guidance: weigh flour whenever a recipe gives grams. When it gives cups, spoon and level and use 120 g per cup, 60 g per half cup, 30 g per quarter cup and 7.5 g per tablespoon; for an America's Test Kitchen recipe use 142 g per cup. A 5 lb (2.27 kg) bag holds about 19 spooned cups.