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Physical unit
Unit name calorie
Unit symbol
Physical quantity (s) Energy , work , internal energy , warmth
In SI units

Calorie ( unit symbol cal ) is an outdated unit of measurement of energy , especially the amount of heat . According to a common definition, a calorie is the amount of heat that is required to (under certain conditions) heat 1 gram of water by 1 degree Celsius (see below).

In 1948 the 9th General Conference on Weights and Measures in Paris replaced the calorie, the physical unit of heat, with the unit joule . One calorie corresponds to approx. 4.1868 joules (1  k cal = 4.1868  k J) and one joule approx. 0.239 calories. However, due to different definitions of calorie, slightly different values ​​exist.

In Germany, the use of joules as an international SI unit of energy has been required by law since 1969 for official information and business transactions. According to the latest guideline from 2010, in the movement of goods in the EU, in addition to information in joules as the unit of energy, an additional information in the unit calorie is permitted. In the case of food, this additional information may only be given in accordance with the Food Information Ordinance (LMIV) with the unit kilocalorie ( kcal ) and in brackets after the SI unit kilojoule (kJ). For example: calorific value 210 kJ / 100 g (50 kcal / 100 g).

Word origin

The word comes from French: Calorie , from which the unit symbol is derived. It is derived from the Latin calor (heat).


In principle, all definitions of calories refer to the amount of heat that is required to heat a certain mass of water, 1 gram or 1 kilogram, by 1 Kelvin or 1 ° C, i.e. the specific heat capacity of water. However, this value is clearly temperature-dependent ; it also depends on the ambient pressure and the type of water used (chemical purity , isotopic composition ).

Liquid water has a minimum specific heat capacity at around 30–50 ° C of just under 4.18 kJ · kg −1 · K −1 , at 0 ° C and 100 ° C it is around 4.22 kJ · kg - 1 · K −1 . Accordingly, there are different definitions of calories. Some of them only define the measurement rule with which a value can be determined with a certain measurement accuracy ; others define an exact conversion factor to other units.

At the beginning (middle of the 19th century) a reference to the heating of water from 0 ° C to 1 ° C was common, that is to say a very high value of about 4.22 kJ per kilocalorie (“kilogram calorie”); Later a standardization of the (gram) calorie to exactly 42 million erg , corresponding to 4.2 J, was discussed.

The international standard ISO 80000-5, like its predecessor ISO 31-4, lists 3 definitions of (gram) calories in Appendix B, but disapproves of their use:

15 ° C calorie cal. 15
The amount of heat that is required to heat 1 g of air-free water at a constant pressure of 1013.25 hPa (the pressure of the standard atmosphere at sea level) from 14.5 ° C to 15.5 ° C.
An exact value for this is not provided; 1 cal 15  ≈ 4.1855 J is given as an approximation , with an uncertainty of ± 0.5 mJ.
International table calorie cal IT
"IT" stands for "International (Steam) Table" ("international (steam) table"). The international table calorie is defined as exactly 1 cal IT  = 4.1868 J.
This value was established in 1956 at the 5th international conference on the properties of water vapor in London. It was chosen, among other things, because the sequence of digits can be divided by 18 without a remainder, making it easier to convert heat measurements, which, like the Btu, are based on ° F.
The international table calorie is often confused with the international calorie cal int , which was defined as 1860 of an international watt hour at a previous conference in 1929 (under the name "International Steam Table Conference") . Units with the prefix “international” relate to old definitions of ohms and amperes or volts , which were more practical, but independent of meters and kilograms and therefore differed from the corresponding “absolute” units with increasing measurement accuracy. When the units were reproduced with newer technical means, various national "international" units were created. With the mean values ​​for the international ohms and volts of 1,00049 Ω and 1,00034 V determined by the 9th General Conference on Weights and Measures in 1948 , 1 cal int  ≈ 4.18684 J results ; the commonly cited 4.18674 J refer to US international units (1,000495 Ω and 1,000330 V).
Thermochemical calorie cal th
The thermochemical calorie (also called defined or Rossini calorie ) is defined with exactly 1 cal th  = 4.184 J. This value is based on the value of 4.1833 international joules previously used in thermochemical work in the USA , converted using US American international units, and was originally defined by the National Bureau of Standards .
On the basis of the thermochemical calorie, the TNT equivalent was also defined, with which the energy released by nuclear weapons was specified. 1 kilogram of TNT = 1000 kcal th = 4184 kJ
Mean calorie cal
The mean calorie corresponds to the 100th part of the amount of heat required to heat 1 gram of water from 0 ° C to 100 ° C at an atmospheric pressure of 1013 hPa. The derived value of 1 cal = 4.1897 J (for 1 kg of water 1 kcal = 4.1897 kJ) therefore defines the mean specific heat capacity of water as 4.1897 kJ / (kg · K) over the temperature range in which it is liquid.

Other common definitions refer to the specific heat capacity of water at 4 ° C ( maximum density ; approximately 4.204 J) or 20 ° C (reference temperature for liquid food in the EU; approximately 4.182 J); other reference points in the range of typical laboratory temperatures were also common.

The International Union for Nutritional Sciences (IUNS) has decided a value of exactly 4.182 J.


The word “calorie” can refer to a calorie (1 cal) or, strictly speaking, incorrectly, to a kilocalorie (1 kcal). Both uses occur in colloquial language and in the life sciences. To differentiate, one sometimes speaks of “gram calorie” and “kilogram calorie” or “small calorie” and “large calorie”, which each refer to the warming of one gram or one kilogram of water by one degree Celsius . In practice, however, the designation “kilocalorie” for 1000 gram calories is clear, analogous to the prefixes for SI units .

In the past, the output of stoves or heating systems was not given in watts , but in kcal / h.

Use in physics

In physics, after the introduction of the CGS system , the calorie is clearly defined as a gram calorie. Although the amount of heat could be expressed by the CGS unit of energy, erg , and also the joule from the parallel practical electromagnetic system (based on ohms and volts ), there was often a desire to keep a special unit of heat amount, which is on the specific heat capacity of water. Water played a decisive role in the decision in favor of the CGS system and against a system with meters instead of centimeters as the basic unit of length , because its density in the basic units is thus approximately 1. Around 1900 it was initially unclear what the name and the exact definition of such a unit would be, especially the question of whether it should be derived from a fixed numerical value from the joule or erg or defined by its own measurement specification.

Later standardizations of the calorie always took this gram calorie as a basis and defined a fixed numerical value for conversion from other units; measurement-based definitions have also existed. Uniform usage ceased to exist before the calorie was increasingly replaced by the joule.

Use as a nutritional information

The physiological calorific value of food in particular is often given in kilocalories in addition to the kilojoule; From 1990 to the end of 2009, the EU directive on nutrition labeling stipulated that information should be given in both kJ and kcal. On the other hand, in the EU economy and public health the SI units must always be used; they must be highlighted in relation to additionally specified other units. In particular, a statement in kcal must not be set in larger font.

Since January 1, 2010, the calorific value of a food in the EU has to be standardized in kJ related to a certain amount - e. B. in kJ / 100 g - the additional specification of the calorific value in kcal is permitted without a time limit. A deadline for abolishing the additional use of kcal as a unit in the movement of foodstuffs was initially set at the beginning of 1990 and then extended to the beginning of 2000; the last extension to the end of 2009 and the complete abolition of the deadline are justified by trade barriers when exporting to third countries. From the beginning of 1978 (the end of the transition period of the previous directive) to the end of September 1981 (the entry into force of the previous directive), the use of calories was prohibited in the EU.

Colloquially, nutritional information in kilocalories is often incorrectly referred to as "calories". In the USA, the term calorie for kilocalories is also officially permitted in nutritional information .

The recommended daily allowance for adults varies depending on gender and age. On average, it is around 8–13 MJ (2–3 Mcal / 2000–3000 kilocalories), as Carl von Voit had already calculated in the second half of the 19th century, while active high-performance athletes can easily need twice as much.

The Austrian Heavy Labor Ordinance defines heavy physical work and the like. a. so if at least 8374 kilojoules (2000 kilocalories) are consumed by men and at least 5862 kilojoules (1400 kilocalories) are consumed by women during an eight-hour working period ( Section 1  (1) no. 4 Ordinance on Heavy Labor).

Web links

Wiktionary: calorie  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ^ Resolution 3 of the 9th meeting of the CGPM (1948) ( online , accessed June 27, 2020), English
  2. Duden. In: Retrieved August 12, 2016 .
  3. Mean, molar heat capacity , ChemgaPedia
  4. ^ A b History of the Calorie in Nutrition , American Society for Nutrition
  5. Calorie ,
  6. ^ Realization of SI units , Kaye and Laby Online
  7. ^ Electrical Engineers Handbook Electric Power , Harold Pender, William A. Del Mar, 1949
  8. ^ A b The Adoption of Joules as Units of Energy , Food and Agriculture Organization of the United Nations
  9. Peter Kurzweil: The Vieweg Einheit-Lexikon, Vieweg: Braunschweig 1999, p. 67
  10. Directive 75/106 / EEC of the Council of December 19, 1974 on the harmonization of the laws of the member states on the filling of certain liquids by volume in prepackages (PDF)
  11. ^ First Report of the Committee for the Selection and Nomenclature of Dynamical and Electrical Units , in: Report of the Forty-Third Meeting of the British Association for the Advancement of Science ; Held at Bradford in September 1873; London 1874, p. 222
  12. Experiments for improving the Construction of Practical Standards for Electrical Measurements, Report of the Committee, Appendix On the Definition of the Unit of Heat , in: Report of the Seventy-Second Meeting of the British Association for the Advancement of Science ; Held at Belfast in September 1902, London 1903, page 55
  13. ^ Twenty-Third Report - Liverpool, 1896 , in: Reports of the Electrical Standards Committee of the British Association, Cambridge 1913, p. 539
  14. Directive 90/496 / EEC of the Council of September 24, 1990 on the nutrition labeling of foods (PDF)
  15. a b Council Directive 80/181 / EEC of December 20, 1979 on the approximation of the laws of the member states on units in metrology and the repeal of Directive 71/354 / EEC (PDF)
  16. Directive 89/617 / EEC (PDF) of the Council of November 27, 1989 amending Directive 80/181 / EEC on the harmonization of the laws of the member states on units in metrology
  17. Directive 99/103 / EC (PDF) of the European Parliament and of the Council of January 24, 2000 amending Directive 80/181 / EEC on the harmonization of the laws of the Member States on units in metrology
  18. Directive 2009/3 / EC (PDF) of the European Parliament and of the Council of March 11, 2009 amending Directive 80/181 / EEC of the Council on the approximation of the laws of the member states on units in metrology
  19. Directive 71/354 / EEC (PDF) of the Council of October 18, 1971 on the approximation of the laws of the member states on units in metrology
  20. Calorie consumption and good sleep