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of heat, entering French and English dictionaries between 1841 and 1867.[1] In most fields its use is archaic, having been replaced by the SI unit of energy, the joule.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:
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. Despite metrication, the calorie is still often encountered in contexts such as bond and conformational energies in molecular modeling.[3]
The gram calorie, however, is a very small unit for use in nutritional contexts. Instead, the kilocalorie (symbol: kcal) or large calorie is used. In such context calorie and kilocalorie are equivalent.
The energy needed to increase the temperature of a given mass 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 conversion factors used to convert calories to joules are numerically equivalent to expressions of the specific heat capacity of water in joules per gram or kilogram.
| Name | Symbol | Conversions | Notes |
|---|---|---|---|
| Thermochemical calorie | calth | ≡ 4.184 J | [4] |
| 4 °C calorie | cal4 | ≈ 4.204 J
≈ 0.003985 BTU ≈ 1.168×10−6 kWh ≈ 2.624×1019 eV |
the amount of energy required to warm one gram of air-free water from 3.5 °C to 4.5 °C at standard atmospheric pressure. |
| 15 °C calorie | cal15 | ≈ 4.1855 J
≈ 0.0039671 BTU ≈ 1.1626×10−6 kWh ≈ 2.6124×1019 eV |
the amount of energy required to warm one gram of air-free water from 14.5 °C to 15.5 °C at standard atmospheric pressure (101.325 kPa). Experimental values of this calorie ranged from 4.1852 J to 4.1858 J. The CIPM in 1950 published a mean experimental value of 4.1855 J, noting an uncertainty of 0.0005 J.[4] |
| 20 °C calorie | cal20 | ≈ 4.182 J
≈ 0.003964 BTU ≈ 1.162×10−6 kWh ≈ 2.610×1019 eV |
the amount of energy required to warm one gram of air-free water from 19.5 °C to 20.5 °C at standard atmospheric pressure. |
| Mean calorie | calmean | ≈ 4.190 J
≈ 0.003971 BTU ≈ 1.164×10−6 kWh ≈ 2.615×1019 eV |
1⁄100 of the amount of energy required to warm one gram of air-free water from 0 °C to 100 °C at standard atmospheric pressure. |
| International Steam Table calorie (1929) | ≈ 4.1868 J
≈ 0.0039683 BTU ≈ 1.1630×10−6 kWh ≈ 2.6132×1019 eV |
1⁄860 international watt hours = 180⁄43 international joules exactly.[5] | |
| International Steam Table calorie (1956) | calIT | ≡ 4.1868 J
≈ 0.0039683 BTU ≈ 1.1630×10−6 kWh ≈ 2.6132×1019 eV |
1.163 mW·h = 4.1868 J exactly. This definition was adopted by the Fifth International Conference on Properties of Steam (London, July 1956).[4] |
| IUNS calorie | ≡ 4.182 J
≈ 0.003964 BTU ≈ 1.162×10−6 kWh ≈ 2.610×1019 eV |
This is a ratio adopted by the Committee on Nomenclature of the International Union of Nutritional Sciences.[6] |