Calorie

The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867. In most fields its use has been replaced by the SI unit of energy, the joule. However, it remains a commonly used unit for energies in the field of chemistry, and in many countries it remains in common use as a unit of food energy.

Definitions of the calorie fall into two classes:

• The small calorie or gram calorie (symbol: cal) approximates the energy needed to increase the temperature of 1 gram of water by 1 °C at standard atmospheric pressure (101.325 kPa). This is approximately 4.2 joules.
• The large calorie, kilogram calorie, dietary calorie, nutritionist's calorie or food calorie (symbol: Cal) approximates the energy needed to increase the temperature of 1 kilogram of water by 1 °C. This is exactly 1,000 small calories or approximately 4.2 kilojoules.

The calorie, the kilocalorie and the kilojoule are in common use in nutritional contexts as units of food energy. As used in these contexts the calorie (unprefixed) is based on the kilogram whereas the kilocalorie is based on the gram. That is, the nutritional calorie is the kilogram calorie and the kilocalorie is one thousand gram calories. Thus, in nutrition, the terms calorie and kilocalorie refer to equivalent units.

In an attempt to avoid confusion, the large calorie is sometimes written as Calorie (with a capital C). This convention, however, is not always followed, and not explained to the average person clearly (and is sometimes ambiguous, such as at the beginning of a sentence). Whether the large or small calorie is intended often must be inferred from context. When used in scientific contexts, the term calorie refers to the small calorie; it is often encountered in experimental calorimetry, and commonly used to specify bond and conformational energies in molecular modeling.