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A calorie (abbreviated cal) is the amount of heat necessary to raise the temperature of one gram of water by 1 degree Celsius, at a pressure of 1 atm. This amount of heat depends somewhat on the initial temperature of the water, which results in various different units called "calorie":

  • the 15 °C calorie,
  • the 4 °C calorie,
  • the mean 1 °C to 100 °C calorie,
  • the International Steam Table calorie,
  • the thermochemical calorie,

Of these, the 15 °C calorie is what is most commonly meant by calorie in contemporary English text.

These units are now deprecated. The SI unit for heat (and all other forms of energy) is the joule (J), while the (obsolete) cgs system uses the erg.

One 15 °C calorie is the amount of heat necessary to raise the temperature of 1 g of water from 14.5 °C to 15.5 °C. This is approximately equal to 4.1855 J or 3.968×10-3 Btu. The International Table calorie is approximately equal to 4.1868 J and the thermochemical calorie 4.184 J.

Nutritionists, when describing the energy content of food, typically refer to Calories (capitalized and abbreviated as C or kcal); one Calorie equals 1000 15 °C calories, or about 4,186 J. The energy content of fat is 9 kcal/g. of proteins and carbohydrates 4 kcal/g.

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