calorie, abbr. cal, unit of heat energy in the metric system. The measurement of heat is called calorimetry. The calorie, or gram calorie, is the quantity of heat required to raise the temperature of 1 gram of pure water 1°C;. The kilocalorie, or kilogram calorie, is the quantity of heat required to raise the temperature of 1 kg of pure water 1°C;; it is equal to 1,000 cal. The kilocalorie is used in dietetics for stating the heat content of a food, i.e., the amount of heat energy that the food can yield as it passes through the body; in this context, the kilocalorie is usually called simply the calorie. The amount of heat energy needed to effect a 1°C; temperature increase in 1 gram of water varies with temperature (see heat capacity); thus the temperature range over which the heating takes place must be stated to define the calorie precisely. The 15° calorie, or normal calorie, is widely used in chemistry and physics; it is measured by heating a 1-gram water sample from 14.5°C; to 15.5°C; at 1 atmosphere pressure. The 4° calorie, also called the small calorie or therm, is measured from 3.5°C; to 4.5°C; (water is most dense at 3.98°C;); the large calorie, or Calorie, is equivalent to 1,000 small calories. The average value of the calorie in the range 0°C; to 100°C; is called the mean calorie; it is 1/100 of the energy needed to heat 1 gram of water from its melting point to its boiling point. The calorie may also be defined by expressing its value in some other energy units. The 15° calorie is equivalent to 4.185 joules (J), 1.162×10^-6 kilowatt-hours, 3.968×10^-3 British thermal units, and 3.087 foot-pounds; the 4° calorie equals 4.204 J; and the mean calorie equals 4.190 J. Two other calories sometimes used are the International Steam Table calorie, equal to 4.187 J, and the thermochemical calorie, equal to 4.184 J. When the calorie is used for precision measurement of heat energy, the particular calorie being used must be specified.

Regulating food intake to improve physical condition, especially to lose weight. Examples include diets low in fat for weight loss, low in saturated fat and cholesterol to prevent or help treat coronary heart disease, or high in carbohydrates and protein to build muscle. Weight-loss diets are based on reducing calorie intake in different proportions of fat, carbohydrate, or protein; most result in some weight loss, but often the weight is gained back within a few years. Diets must include adequate nutrition and are most effective combined with exercise. Appetite suppressants may have dangerous side effects. Excessive weight loss may be a sign of anorexia nervosa.

Learn more about dieting with a free trial on Britannica.com.

Unit of energy or heat. Various precise definitions are used for different purposes (physical chemistry measurements, engineering steam tables, and thermochemistry), but in all cases the calorie is about 4.2 joules, the amount of heat needed to raise the temperature of 1 g of water by 1 °C (1.8 °F) at normal atmospheric pressure. The calorie used by dietitians and food scientists and found on food labels is actually the kilocalorie (also called Calorie and abbreviated kcal or Cal), or 1,000 calories. It is a measure of the amount of heat energy or metabolic energy contained in the chemical bonds (see bonding) of a food.

Learn more about calorie with a free trial on Britannica.com.

This article is about the unit of energy. For its use in nutrition and food labelling regulations, see the article on food energy.

The calorie is a pre-SI unit of energy, in particular, heat. In most fields, its use is archaic, and the SI unit of energy, the joule, has become accepted. However, it remains in common use as a unit of food energy. It was first defined by Professor Nicolas Clément in 1824 as a kilogram-calorie, and this definition entered French and English dictionaries between 1841 and 1867. Etymology: French calorie, from Latin calor (heat).

The unit calorie has historically been used in two major alternate definitions that differ by a factor of 1000:

• The small calorie, gram calorie, or calorie (symbol: cal) is the amount of heat (energy) required to raise the temperature of one gram of water by 1 °C.
• The large calorie, kilogram calorie, kilocalorie (symbol: kcal), or Calorie (capital C) is the amount of heat (energy) needed to increase the temperature of one kg of water by 1 °C, exactly 1000 small calories, or about 4.184 kJ.

The second definition is the one commonly used to express food energy, e.g. when discussing dieting or nutrition plans. Under this definition, 1 g of pure carbohydrate yields about 4 Calories of energy, and the recommended intake for an adult person is about 2,000 - 2,500 Calories/day. It is alternatively referred to as Calorie (Cal), kilocalorie (kcal) or even calorie with lowercase 'c'. The potential for confusion can be avoided by using the SI units (joules or kilojoules).

Apart from these two major alternate definitions, there exist also minor variants of the definition of this unit, which differ in the exact experimental conditions used, most notably the start temperature of the water (see section below).

The factors used to convert measurements in calories to their equivalents in joules are numerically equivalent to expressions of the specific heat capacity of water in SI units. See "Versions" below for an explanation of the units.

1 cal_IT = 4.1868 J (1 J = 0.23885 cal_IT) (International Steam Table calorie, 1956)

1 cal_th = 4.184 J (1 J = 0.23901 cal_th) (Thermochemical calorie)

1 cal₁₅ = 4.18580 J (1 J = 0.23890 cal₁₅) (15°C calorie)

Versions

The energy needed to increase the temperature of a gram of water by 1 degree Celsius depends on the starting temperature and is difficult to measure precisely. Accordingly, there have been several definitions of the calorie:

• Thermochemical calorie (cal_th): 4.184 J exactly.
• 15 °C calorie (cal₁₅): the amount of energy required to warm 1 g of air-free water from 14.5 °C to 15.5 °C at a constant pressure of 101.325 kPa (1 atm). Experimental values of this calorie ranged from 4.1852 J to 4.1858 J. The CIPM in 1950 published a mean experimental value of 4.1855 J, noting an uncertainty of 0.0005 J.
• 20 °C calorie: the amount of energy required to warm 1 g of air-free water from 19.5 °C to 20.5 °C at a constant pressure of 101.325 kPa (1 atm). This is about 4.182 J.
• 4 °C calorie: the amount of energy required to warm 1 g of air-free water from 3.5 °C to 4.5 °C at a constant pressure of 101.325 kPa (1 atm).
• Mean calorie: 1/100 of the amount of energy required to warm 1 g of air-free water from 0 °C to 100 °C at a constant pressure of 101.325 kPa (1 atm). This is about 4.190 J
• International Steam Table Calorie (1929): (1/860) W h = (180/43) J exactly. This is approximately 4.1868 J.
• International Steam Table Calorie (1956) (cal_IT): 1.163 mW h = 4.1868 J exactly. This definition was adopted by the Fifth International Conference on Properties of Steam (London, July 1956).
• IUNS calorie: 4.182 J exactly. This is a ratio adopted by the Committee on Nomenclature of the International Union of Nutritional Sciences.

The two perhaps most popular definitions used in older literature are the "15 °C calorie" and the "thermochemical calorie". Since the many different definitions are a source of confusion and error, all calories are now deprecated in favour of the SI unit for heat and energy: the joule (J).

References

See also

• Food energy
• Basal metabolic rate
• Nutrition facts label