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 In most fields its use is archaic, having been replaced by the SI unit of energy, the joule. However, in many countries it remains in common use as a unit of food energy.
Definitions of a 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. This is about 4.2 joules.
- The large calorie, kilogram 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 1000 small calories or about 4.2 kilojoules.
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 is sometimes impossible). 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 gram calorie. In nutritional contexts, however, a larger unit is more useful. In such contexts the term calorie can be taken to refer to the kilogram calorie. The term kilocalorie (symbol: kcal) can also generally be taken to refer to 1000 gram calories since metric prefixes tend to be used only with the gram calorie.
The energy needed to increase the temperature of a gram of water by 1 °C depends on the starting temperature and is difficult to measure precisely. Accordingly, there have been several definitions of the calorie. The two perhaps most popular definitions used in older literature are the 15 °C calorie and the thermochemical calorie.
The factors used to convert measurements in calories to their equivalents in joules are numerically equivalent to expressions of the specific heat capacity of water in joules per gram or kilojoules per kilogram.
The conversion factor between calories and joules is numerically equivalent to the specific heat capacity of liquid water (in SI units).
One gram calorie is approximately:
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