The calorie is a unit of energy that originated from the caloric theory of heat. The small calorie or gram calorie is defined as the amount of heat needed to raise the temperature of one milliliter of water by one degree Celsius (or one kelvin). The large calorie, calorie of food, dietary calorie, kilogram calorie, or kilocalorie (kcal) is defined as the amount of heat needed to raise the temperature of one liter of water by one degree Celsius. Thus, 1 large calorie is equal to 1,000 small calories (1 kcal = 1000 cal).