Serious bakers know that flour is easily compacted or aerated depending on how it is handled.
If you dip a measuring cup into a flour container and sweep across the cup to even out the measurement – called dip and sweep – the measurement can change with the force of the scoop which will cram more or less flour into the cup. Estimates of this measuring method give anywhere from 4 to 6 ounces of flour per cup.
If you use the spooning method, which means you spoon the flour from the container into the cup and then level it off, there will be less compression of the flour and it will weigh about 4 ½ ounces per cup.
If you are measuring flour with a digital kitchen scale, use 5-ounces of flour per cup if the recipe you are using provides only measurements and not weight.