Ask the Nutritionist: Are the Calorie Counts Correct?
To be honest, the calories listed on a nutrition label are more of an estimate than a strict number – but they're pretty darn close together.
A calorie is quantified as the amount of energy required to raise the temperature of 1 milliliter of water by 1 degree Celsius. Scientists place a single whole food in a calorimeter – a device with a combustion chamber, thermometer, and water container – and burn it. The rise in water temperature will help determine the approximate number of calories in this food.
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Most manufacturers use existing databases (rather than running tens of millions of mini bunsen burner fires) to determine the number of calories in each ingredient of a product, which they then add up to get their nutritional information. To calculate the calories of your own recipes, visit calorieking.com or the U.S. Department of Agriculture's Food Nutrient Profiles database – FoodData Central – for your totals (fairly accurate).