calorie
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 Clement 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 form is the one commonly used to express food energy. Its most common name is  calorie; kilocalorie is sometimes used, more often in the symbol "kcal" than in the spelled out word.
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.

# 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 (calth): 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) (calIT): 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).