Whole wheat flour is flour that still contains the wheat germ and bran (as opposed to white flour, which is ground from only the wheat's starchy endosperm). As a result, whole wheat flour has more fiber and nutrients than all-purpose white flour, and whole wheat baked goods have a more interesting flavor and chewier texture.
1 cup whole wheat flour = 4-1/2 oz.
If recipe calls for whole-wheat flour and all you have is all-purpose, try using an extra 1 tablespoon per cup. If you want to experiment with substituting whole wheat flour in recipes that call only for white flour, start out by swapping out just a small portion of the flour. You can often substitute up to half of the total flour with whole wheat and still get good results. Too much whole wheat flour, though, can give your bread a leaden texture, because it is very high in gluten-forming protein.
When measuring whole wheat flour, use the same technique as for regular flour: stir lightly to aerate, then scoop the flour into the measuring cup, then use a knife to level it off. Or for even more accuracy, measure by weight.
Keep whole wheat flour refrigerated, because the whole grain will go rancid more quickly at room temperature.