In both physics and nutrition, a calorie is a unit of energy measurement. The dietary Calorie is based on the physics calorie, but it has a more limited, specific application and is not used as precisely. In physics, there are two types of calorie: a gram calorie is the amount of energy required to raise the temperature of a gram of water by one degree Celsius, while a kilogram calorie is the amount of energy needed to increase the temperature of a kilogram of water by one degree Celsius. The dietary calorie is based on the latter measurement in physics.

In physics, the term calorie is only used to refer to the gram calorie, while the dietary Calorie is really a kilogram calorie, or kilocalorie, 1,000 times larger than a physics calorie. (The dietary calorie should be written as Calorie, with a capital "C".) This is only one way in which the two differ. Dietary Calories are also much more approximate in nature than physics calories.

The idea behind a dietary calorie is that the body must balance the energy consumed with the energy expended in order to manage weight. Weight loss can be achieved by increasing one's energy output through physical exercise while decreasing one's energy intake through monitoring the diet. Dietitians, following physics terminology, use the Calorie to measure this energy exchange. Dietary Calories are based on the amount of energy the body can absorb from each food type, rather than on the amount of physics calories inherent in the food.

Dietary Calories are calculated based on standardized values approximated for different food components. Fat, for example, is estimated to have nine dietary Calories per gram, while sugar-free sweeteners such as sorbitol have only two and a half Calories per gram. Protein and carbohydrates are estimated to have four Calories per gram. Some ingredients in food, such as water, vitamins, and minerals, do not have Caloric content and are not taken into account in this measurement.

Nutritional guidelines generally suggest that adult women consume 2,000 dietary Calories per day and adult men consume 2,500. Children, people with sedentary lifestyles, older people, and those trying to lose weight should consume fewer Calories than these guidelines recommend, while very active individuals should consume more. These guidelines are very general and do not apply to every person. If you are unsure about the amount of dietary Calories you should be consuming daily, consult a doctor or nutritionist.