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 kcalcal); 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.

This thousand-fold difference has lead to a joking calculation, sometimes taken for real, which "shows" that warming cold beer (or ice cream) in the stomach requires more energy than present in the beer, and thus cold beer can be used for losing weight. The error lies in the conversion of nutritional beer "Calories" to thermochemical calories instead of kcal, in effect even frozen beer contains much more nutritional energy than required to warm it to body temperature. (see Beer and Ice Cream Diet)