The calorie (symbolized cal) is a unit of heat occasionally used in the centimeter-gram-second ( cgs ) system of physical units. Heat is a form of kinetic energy transfer from one medium or object to another.
When 1 cal of heat energy is transferred to one gram (1 g) of pure liquid water, the temperature of that sample of water is raised by one degree Celsius (1º C) or one degree Kelvin (1º K), provided the water temperature is above the freezing point and below the boiling point. When the temperature of 1 g of pure liquid water falls by 1º C or 1º K, that water gives up 1 cal of heat energy. (The Celsius degree and the Kelvin degree represent a temperature increase or decrease of the same amount, although the scales are offset by 273.15.) One gram of pure liquid water has a volume of one centimeter cubed (1 cm3) or one milliliter (1 ml).
In nutrition, a unit called the kilocalorie, also known as the diet calorie, is frequently mentioned. This unit is equivalent to 1000 cal, and is the amount of heat energy required to raise or lower one kilogram (1 kg) of pure liquid water by 1º C or 1º K. When the label on a package of food says that a serving contains 200 calories, it means that the sample would yield 200 kcal (not 200 cal) of heat energy if subjected to complete combustion. Kilocalories are used (and misused) to express the amount of food consumed in a serving, a meal, or a day. It has been said that a surplus of 7700 kcal of food intake will result in a mass gain of 1 kg, or that a surplus of 3500 kcal of food intake will produce a weight gain of one pound (1 lb), which represents 0.454 kg in the gravitational field of the earth at the surface. Some nutritionists and physicians, however, point out that this is a simplistic view, and can lead to abusive dieting and the neglect of physical activity as a factor in the maintenance of ideal weight and overall health.
In physics, the standard unit of energy or work is the joule (symbolized j). One calorie is the equivalent of approximately 4.1868 j. Conversely, 1 j is the equivalent of approximately 0.23885 cal. Other, less common, units of energy include the British thermal unit ( Btu ) and the erg . The Btu is equivalent to approximately 1055 j, and the erg is equivalent to exactly 0.0000001 (10-7) j. The Btu is sometimes used in reference to residential and commercial heating and air conditioning systems. The erg is occasionally specified when it is necessary to express extremely small quantities of energy.