A kilocalorie is the scientific term for what most of us call a calorie. Technically, a kilocalorie is 1,000 calories, but the term calorie is generally used instead of kilocalorie to describe the units of energy in food, or food calories. So when a food label shows that one serving has 200 calories, it really means 200 kilocalories. Most foods today come with the full nutritional label detailing their calorie counts, macronutrient counts, fiber content and more. If a food item does not have this information, you can use a book, website or database to find it, but either way, you will still have to do a little calculating yourself to get the values for the amount of food you consumed.
Determine the individual ingredients in your food item. When you're eating something simple like a carrot, there is only one ingredient, but for a more complex food, like a tuna fish sandwich, there might be several. As an example, let's dissect a tuna fish sandwich and find out how many calories it might have. The ingredient list for the tuna sandwich will be: two slices of wheat bread, one six-ounce can of tuna packed in water, two tablespoons of mayonnaise, and one tablespoon of pickle relish.
Find the calories for a standard serving of each of the ingredients in your selected food. To keep things simple, we will use the online USDA nutrient database for our values, but you could also use the package labels or another source for this information. Here are the ingredients for a tuna sandwich, in their standard serving sizes from the database, with the associated calories: wheat bread, one slice, 70 calories; tuna packed in water, 1 ounce, 33 calories; mayonnaise, 1 tablespoon, 99 calories; pickle relish, 1/2 cup, 111 calories.
Convert the calories from the standard serving to the amount you are consuming. To do this, you must multiply the calories from the standard serving size by your serving size. For example, the standard serving for wheat bread is one slice, but your sandwich calls for two slices, so you will multiply the standard serving's 70 calories by two, to give 140 calories from wheat bread in your sandwich. Likewise, the standard serving for the tuna is one ounce, but you are using six ounces, so your tuna calories = 6 x 33 = 198 calories. The mayonnaise serving is already what you'll use, so no conversion is necessary. Finally, you need only one tablespoon of pickle relish, but the database supplied a half-cup measure, which is eight tablespoons, so you will have 111 / 8 = 14 calories from relish.
Sum up the calories from the individual ingredients. For our tuna sandwich, that is bread + tuna + mayo + relish = 140 + 198 + 99 + 14 = 451 total calories or kilocalories.
Things You'll Need
List of ingredients in your selected food
Nutrition information, either from database, website or package label
If you are having a hard time converting the product label's serving size to what you are actually using, consider using the USDA online database, as it has a number of different measurements for each type of food. For example, the label on a package of butter may only give you the calories per tablespoon, but you may only be using a "pat" of butter or an odd amount like one-third cup, which is a pretty tricky conversion. The USDA database, however, lists butter not only in tablespoons, but also in grams, sticks, cups, and pats, so it could help you avoid those crazy conversions.
If you calculate the total calories for a sandwich or a bowl of soup or a plate of spaghetti, but then don't eat it all, you'll need to do one more calculation. In this case, try to estimate what portion of the whole thing you did consume, and then take that percentage of the total calories. For example, if you only ate three-quarters of the tuna sandwich above, your total calories would be 3/4 x 451 = 338.