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A unit of energy. Defined as the amount of heat needed to raise the temperature of 1 gram of water (at one atm) by 1 Celsius.

The 15° calorie, or normal calorie, is measured by heating 1 gram water from 14.5 °C to 15.5 °C at 1 atmosphere absolute pressure. The 4° calorie, also called the small calorie, is measured from 3.5 °C to 4.5 °C (water is most dense at 3.98 °C); the large calorie, or Calorie, is equal to 1000 small calories. The Calorie is used in food science to give the energy content of food.

The calorie may also be defined by expressing its value in SI units. The 15° calorie is equivalent to 4.185 joule, the SI unit for energy. Two other calories sometimes used are the International Steam Table calorie, equal to 4.187 J, and the thermochemical calorie, equal to 4.184 J. When the calorie is used for precision measurement of energy, the particular calorie being used must be specified.