The two cups in this photo are exactly the same size. The left (red) one contains flour that I aerated, scooped into the cup, and leveled off with the back of a knife. The right (blue) one I did NOT aerate, just scooped it into the cup, and leveled it off with the back of a knife. Then I weighed the flour from both cups.
You can see that the flour that was aerated (red) weighs less than the NOT aerated flour (blue) and that’s because flour settles in the bag or container and if you measure it without aerating, you will probably be using too much flour. This can cause baking problems, especially if you consider that 3 cups of flour, that should weigh 13 1/2 ounces might weigh 15 3/4 ounces if not aerated before measuring. That’s exactly 1/2 cup more flour than you need!
Oh… one more thing. Aerating is not sifting. To aerate flour, you simply stir it around in the container with a spoon or spatula to incorporate some air. You can see how I aerate flour in my Quick and Easy Chocolate Cake video at the one minute mark: Click here to see it. I hope this is helpful and you have great results with all your baking.