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The calorie is a pre-SI metric unit of energy. The unit was first defined by Professor Nicolas Clément in 1824 as a unit of heat. This definition entered French and English dictionaries between 1841 and 1867.[1] In most fields its use is archaic, having been replaced by the SI unit of energy, the joule.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. The kilocalorie per mole remains in use in computational chemistry and molecular spectroscopy.

Definitions vary but are all based on the specific heat capacity of water. The gram calorie, approximately 4.2 J, is based on one gram of water. The kilogram calorie, equal to one thousand gram calories, is based on one kilogram of water. In the context of nutrition, and especially food labelling, a larger unit is used and referred to interchangeably by the terms calorie (or Calorie) and kilocalorie.



Historically, the calorie has had two major alternative definitions differing by a factor of one thousand. In addition to these two major alternative definitions, minor variants of the definition of this unit also exist differing in the exact experimental conditions used, most notably the start temperature of the water.


Kilogram and gram calories

The original definition by Clément was based on the kilogram. Other definitions based on the gram have since been made. We thus have the two major variants: the kilogram calorie and the gram calorie. One thousand gram calories equal one kilogram calorie.

In the context of food energy the term calorie generally refers to the kilogram calorie. However, the term kilocalorie (kcal), referring to one thousand gram calories, is also in widespread use especially by professional nutritionists (when speaking in terms of calories rather than joules). To avoid confusion, the prefix kilo- is not used with the kilogram calorie.

Kilogram calorie
The kilogram calorie, large calorie, food calorie, Calorie (capital C) or just calorie (lowercase c) is the amount of energy required to raise the temperature of one kilogram of water by one degree Celsius.
Gram calorie
The gram calorie, small calorie or calorie (cal) is the amount of energy required to raise the temperature of one gram of water by 1 °C. The gram calorie was once commonly used in chemistry and physics.
Pound calorie
The pound calorie is the amount of energy required to raise the temperature of one pound of water by 1 °C.[citation needed] The pound calorie is encountered in engineering usage for heat transfer calculations and is equivalent to 1.8 British thermal units (1.899 kJ). It is something of a hybrid unit, rarely used now, but may be found for example in older but still useful, engineering (and other) textbooks on heat transfer and heat exchanger design. A heat transfer coefficient may be given in the units lb-cal/(h·ft2·°F).


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.

Name Symbol Equivalent in Joules Notes
Thermochemical calorie calth ≡ 4.184 J [2]
4 °C calorie cal4 ≈ 4.204 J 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 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.[2]
20 °C calorie cal20 ≈ 4.182 J 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 1100 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 1860 international watt hours = 18043 international joules exactly.[3]
International Steam Table calorie (1956) calIT ≡ 4.1868 J 1.163 mW·h = 4.1868 J exactly. This definition was adopted by the Fifth International Conference on Properties of Steam (London, July 1956).[2]
IUNS calorie ≡ 4.182 J This is a ratio adopted by the Committee on Nomenclature of the International Union of Nutritional Sciences.[4]


One gram calorie is approximately:

One kilogram calorie (food calorie) is approximately:

  • 4.184 kJ
  • 3.964 BTU
  • 0.001163 kW·h
  • 2.611×1022 eV

In Grammar

In colloquial English grammar, calories are one of the few units of measurement that are often referred to as a concrete noun. Often calories are spoken as if they were an ingredient in food, or have a physical attribute. While technically this reference is grammatically correct, it should be noted that there is no such thing as a physical calorie, it is solely a unit of measurement.

Notes and references

  1. ^ Etymology: French calorie, from Latin calor meaning "heat".
  2. ^ a b c International Standard ISO 31-4: Quantities and units – Part 4: Heat. Annex B (informative): Other units given for information, especially regarding the conversion factor. International Organization for Standardization, 1992.
  3. ^ Figure depends on the conversion factor between international joules and absolute (modern) joules. Using the mean international ohm and volt (1.00049 Ω, 1.00034 V [1]), the international joule is about 1.00019 J, using the US international ohm and volt (1.000495 Ω, 1.000330 V) it is about 1.000165 J, giving 4.18684 J and 4.18674 J, respectively
  4. ^ FAO (1971). "The adoption of joules as units of energy". "While the nutritional calorie has not been defined, basically it is the thermochemical calorie. The standards used in calorimetric work in nutrition is ultimately the heat of combustion of an internationally graded standard benzoic acid. This is primarily expressed as joules per gramme mole and secondarily as thermochemical calories per mole derived by dividing by 4.182, a factor which has been approved by the Committee on Nomenclature of the IUNS." 

See also


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