Flour substitutes

"Cake flour has a 6-8% protein content and is made from soft wheat flour. It is chlorinated to further break down the strength of the gluten and is smooth and velvety in texture. Good for making cakes (especially white cakes and biscuits) and cookies where a tender and delicate texture is desired. To substitute cake flour for all-purpose flour use 1 cup plus 2 tablespoons cake flour for every cup of all-purpose flour. Make your own - one cup sifted cake flour can be substituted with 3/4 cup (84 grams) sifted bleached all-purpose flour plus 2 tablespoons (15 grams) cornstarch."

Read more: http://www.joyofbaking.com/flour.html#ixzz1a8dVEYmZ

So Joy of Baking says use 1 cup plus 2 tablespoons cake flour for every cup of all-purpose flour.

So, if you wanted to MAKE cake flour, I love this: http://www.joythebaker.com/blog/2009/09/how-to-make-cake-flour/

If you don't, my rule of thumb is 1 cup of all purpose flour minus 2 tablespoons is equivalent to 1 cup of cake flour because AP tends to be a bit denser and has a higher protein content that you want to decrease for the more tender crumb