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 form is the one commonly used to express food energy, e.g. when discusing dieting or nutrition plans. Its most common name is calorie; kilocalorie is sometimes used. It is more commonly spelled with the symbol "kcal" than in the spelled out form. While it should properly be written with a capital C, it rarely is, which often leads to confusion that can be altogether avoided when the proper SI unit (Joules or kiloJoules) is used.

SeeWikipedia:Calorie for full article

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