All-purpose flour is, well, an all-around good flour to use for baking breads, cakes, muffins, and for mixing up a batch of pancake batter. All-purpose has protein content of 10-13% and it will perform very well, time after time. But if you want to make really soft cake layers, reach for cake flour. Cake flour has 8-9% protein, making it the weakest flour on the shelf, and it bakes up into meltingly tender cake layers.
To substitute cake flour for all-purpose flour, the most accurate way to do it is to pull out the kitchen scale and substitute it ounce for ounce. All-purpose flour weighs about 4.5 ounces per cup while cake flour weights about 4 ounces per cup. What? Wait a minute. Everyone knows that 1 cup is 8 ounces, so how can 1 cup of all-purpose flour be only 4.5 ounces? This is a common area of confusion, so let’s clear it up. If you fill a 1 cup dry measuring cup with water, it will weigh 8 ounces. But flour weighs less than water, so 1 dry measuring cup of all-purpose flour only weighs 4.5 ounces.
Back to calculating the substitution: If your recipe calls for 2-1/2 cups of all-purpose flour, or 11.25 ounces (2.5 cups x 4.5 ounces=11.25 ounces), weigh out enough cake flour to equal 11.25 ounces. If you measured the cake flour by volume it would equal 2 3/4 cups plus 1 tablespoon.