A question I get asked on a regular basis is “Can I substitute all purpose flour for cake flour?” It is far less common to hear the reverse (although I have actually done so in recipes when I was out of flour and couldn’t be bothered to go to the store), since if you have one kind of flour at home, it will generally be all purpose.
The answer to the question is yes, but the substitution is not one-to-one. The general rule of substitution is 1 cup of all purpose flour minus 2 tablespoons (1c – 2tbsp) is equivalent to 1 cup of cake flour. If you want to substitute cake flour for all purpose, use 2 cups of cake flour plus 2 tablespoons (2 cups + 2 tbsp). If you do decide to use this sub, treat the all purpose flour just as you would cake flour (sifting before measuring, etc.).
Now, that said,I would highly recommend that you stick with whatever type of flour your recipe calls for, even when it means going out to the store to buy a box. The substitution is fairly reliable, but this is definitely a “substitute at your own risk” sort of thing. Different flours have different protein (gluten) contents and different weights, and different flours can result in dramatically different results in different recipes. All purpose flour has about 11% protein content, while cake flour has 6-8%. Some recipes need that low protein content to remain tender and light (like Angel Food Cake) and others are flexible enough (like some cakes or loaves) to use the substitution, but knowing that all purpose flour is so much “stronger” than cake flour should help you decide when you can substitute and when you may not want to.
Note: Some advocate using 1 cup of all purpose flour minus 2 tablespoons plus 2 tbsp cornstarch as a substitute for cake flour. This should work just as well, perhaps better, as the gluten-free cornstarch can cut down on the protein content of the all purpose flour. It still won’t produce results quite as good as cake flour and involves an extra step from the one I mentioned above, so I personally prefer the simpler sub. But going out and buying cake flour (or all purpose) is always going to be your best bet.