Calculate Calories from Food Label

From Harvard's on line class on Science and Cooking.

Calories = 4 X number of grams of protein in a serving plus 4 X number of grams of carbohydrate in a serving plus 9 x the number of grams of fat in a serving 

4 x 49 x 18

whenever I find a label in which the number of the calorie reported disagrees with this calculation, I tend to distrust the label. 

So let me just show you how this works. So here is a label which actually comes from my favorite recipe, which is the recipe for Nestle's Toll House chocolate chip cookies. So it turns out that in one serving of a Nestle Toll House chocolate chip cookie, there are 4 grams of protein, 49 grams of carbohydrate, and 18 grams of fat. So if we want to calculate the calorie content in one serving of the cookie, what we need to do is take the grams of protein, multiply by 4, take the grams of carbohydrate, multiply by 4, take the grams of fat, and multiply by 4. So that's 4 times 4 plus 49 times 4 plus 18 times 9 -- actually sorry, I misspoke. The grams of fat is multiplied by 9, hence the 4-4-9 rule. So we're multiplying 18, which is the grams of fat, by 9. And when we add all of this up, we find that the calorie content of a cookie is 378 calories, which is very close to that which is reported on the label.