The weight of a cup of flour depends on how it is measured. In the test kitchen, we favor the dip-and-sweep method—we dip a measuring cup into a container of flour and then level the flour with a straight edge—some bakers prefer to spoon flour directly into the cup. Since these techniques—along with the heavy- or light-handedness of the baker—incorporate different amounts of air, we’ve found that there can be up to a 20 percent difference in the weight of a cup of flour, a variance that can easily ruin a recipe. To determine our standard weight for flour, we had dozens of volunteers measure out 1 cup, weighed the results, and took the average (5 ounces) as our standard.
When measuring flour, there are various scooping techniques that can lead to a slightly more packed or less packed measuring cup of flour. What's always best is to try to follow the measuring technique of a the recipe developer you are looking at. So for our recipes, and the technique we use to measure a cup of flour, we found that on average it weighs 5 ounces.