Every flour is created equal, right? Not necessarily. The main difference in wheat flours found on the grocery store shelves is the amount of protein they contain. The higher the percentage of protein found in the flour, the greater the strength.
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.