Agricultural chemist Wilbur Atwater determined the calories in most foods a century ago by burning them and measuring the heat they gave off. When the USDA determines the calories in foods today, they do so with sophisticated instruments, but still use conversion tables developed by Atwater and his calorimeter one hundred years ago. The USDA lists the nutrition information for almost every food imaginable on their website, but you can calculate the calories in your meals yourself by learning to read nutrition labels.
For Packaged and Prepared Foods
Step 1
Determine the serving size by looking at the top of the nutrition label. Serving sizes are usually listed in ounces and sometimes units such as "one slice" or "½ bar." If your food is not packaged, you can find an online nutrition label at the USDA website, or online databases like Nutrition Data.
Step 2
Determine the servings per container. The number of servings per container is listed near the serving size at the top of the label. Once you know the serving size and how many servings there are per container, you should be able to estimate how many servings you plan to eat. For example, if a bag of chips is four servings, and you usually eat half the bag, then you are eating two servings (4 servings per bag * ½ bag = 2 servings).
Step 3
The calories contained in each serving are listed near the top of the nutrition label, beneath the words "Amount Per Serving." Multiply the calories per serving by the number of servings you will eat to find out how many calories are in your meal.
For Recipes
Step 1
With a recipe, you already know the amount you will use of each ingredient. Use the nutrition facts on the packaging or an online nutrition facts tool to determine the total calories in each ingredient.
Step 2
Add the calories in the individual ingredients together. This will tell you how many calories are in the whole dish.
Step 3
Decide how many servings you are preparing. For example, one cake mix could yield 12 cupcakes or eight slices of cake.
Step 4
Divide the total number of calories in the dish by the number of servings. For example, if there are 1,200 calories in a tureen of soup that serves a family of four, then each serving is 300 calories (1200 calories ÷ 4 servings = 300 calories per serving).
Tips and Warnings
- When calculating the calories in a recipe, remember to look at the serving size of each ingredient and adjust your calorie count to reflect the total amount you will be using. For example, the serving size for butter is one tablespoon, but the recipe may call for the whole stick. There are 8 tablespoons in a stick of butter, so multiply the calories per serving by eight to determine the total calories in a stick of butter.
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