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.

Definitions

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 (calth)

15 °C calorie (cal15)

20 °C calorie

4 °C calorie

Mean calorie

International Steam Table Calorie (1929)

International Steam Table Calorie (1956) (calIT)

IUNS calorie

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 energy: the joule.

References

1. ^ a b c d International Standard ISO 31-4: Quantities and units – Part 4: Heat. Annex B (informative): Other units given for information, especially regarding the conversion factor. International Organization for Standardization, 1992.
2. ^ FAO (1971). “The adoption of joules as units of energy”. ”While the nutritional calorie has not been defined, basically it is the thermochemical calorie. The standards used in calorimetric work in nutrition is ultimately the heat of combustion of an internationally graded standard benzoic acid. This is primarily expressed as joules per gramme mole and secondarily as thermochemical calories per mole derived by dividing by 4.182, a factor which has been approved by the Committee on Nomenclature of the IUNS.”