In this country, the food manufacturers have already done the legwork for you – you just have to do the math (but they don’t always make is easy). The nutritional facts on the label are based on some serving size and the manufacturer tells you how many serving sizes there are in the package.
For example, a bag of Domino sugar uses 1 teaspoon as the basis for all nutritional calculations. If you know that there are 3 teaspoons in a tablespoon, 4 tablespoons in a quarter cup, and 4 quarter cups in a cup, you can calculate that there are 48 teaspoons in a cup. Then divide the total number of teaspoons in the bag of sugar (567) by 48, and you find that there are 11.8 cups of sugar in that 5-pound bag. So you need all the sugar in a 5-pound bag and just a bit less than 1-1/4 cups more to get to 13 cups.
The flour manufacturers of this world (at least this part of the world) are a little kinder. Their serving size is 1/4 cup, so you divide the number of servings in the package (generally 75 or 76 in a 5-pound bag) by four to get the number of cups in the bag – 18.75 or 19. So you need a little more than 7 pounds of flour for your 27 cups.
We think flour is a little more challenging, though, because it matters a great deal if you just scoop cups of flour out of the bag or whether you fluff the flour, spoon it gently into your measuring cup, and level it off with a straight edge. That’s why professional bakers almost always measure flour by weight. But for the purpose of knowing how much flour to buy, calculations based on the serving size are fine.
Obviously you have to know the relationships between teaspoons, tablespoons, ounces, and fractions of cups to be able to make these kinds of calculations, as manufacturers are not consistent from one product to the next in how they measure a serving.