The calorie is a unit of energy defined as the amount of heat needed to raise the temperature of a quantity of water by one degree. For historical reasons, two main definitions of calorie are in wide use. The small calorie or gram calorie (usually denoted cal) is the amount of heat needed to raise the temperature of one ''gram'' of water by one degree Celsius (or one kelvin). The large calorie, food calorie, or kilocalorie (Cal, calorie or kcal), most widely used in nutrition, is the amount of heat needed to cause the same increase in one ''kilogram'' of water. Thus, 1 kilocalorie (kcal) = 1000 calories (cal). By convention in food science, the large calorie is commonly called Calorie (with a capital C by some authors to distinguish from the smaller unit). In most countries, labels of industrialized food products are required to indicate the nutritional energy value in (kilo or large) calories per serving or per weight. Calorie relates directly to the metric system, and therefore to the SI system. It has been regarded as obsolete within the scientific community since the adoption of the SI system, but is still in some use. The SI unit of energy is the joule. One calorie is defined as exactly 4.184 J, and one Calorie (kilocalorie) is 4184 J.


The calorie was first introduced by Nicolas Clément, as a unit of heat energy, in lectures during the years 1819–1824. This was the "large" calorie, viz. modern kilocalorie. JL Hargrove, "History of the calorie in nutrition", ''J Nutr'' 136/12 (December 2006), pp. 2957–2961. The term entered French and English dictionaries between 1841 and 1867. It comes . The "small" calorie (modern calorie) was introduced by Pierre Antoine Favre (Chemist) and Johann T. Silbermann (Physicist) in 1852. In 1879, Marcellin Berthelot distinguished between gram-calorie (modern calorie) and kilogram-calorie (modern kilocalorie). Berthelot also introduced the convention of capitalizing the kilogram-calorie, as ''Calorie''. The use of the kilogram-calorie (kcal) for nutrition was introduced to the American public by Wilbur Olin Atwater, a professor at Wesleyan University, in 1887. The modern calorie (cal) was first recognized as a unit of the cm-g-s system (cgs) in 1896, alongside the already-existing cgs unit of energy, the erg (first suggested by Clausius in 1864, under the name ''ergon'', and officially adopted in 1882). Already in 1928 there were serious complaints about the possible confusion arising from the two main definitions of the calorie and whether the notion of using the capital letter to distinguish them was sound. Use of the calorie was officially deprecated by the ninth General Conference on Weights and Measures, in 1948. The alternate spelling ''calory'' is archaic.


The modern (small) calorie is defined as the amount of energy needed to increase the temperature of 1 gram of water by 1 °C (or 1 K, which is the same increment). The definition depends on the atmospheric pressure and the starting temperature. Accordingly, several different precise definitions of the calorie have been used. : (a) The 'Thermochemical calorie' was defined by Rossini simply as 4.1833 international joules in order to avoid the difficulties associated with uncertainties about the heat capacity of water. It was later redefined as 4.1840 J exactly. : (b) The figure depends on the conversion factor between "international joules" and "absolute" (modern, SI) joules. Using the mean international ohm and volt (, ), the "international joule" is about , using the US international ohm and volt (, ) it is about , giving and , respectively. : (c) The standard atmospheric pressure can be taken to be . The two definitions most common in older literature appear to be the ''15 °C calorie'' and the ''thermochemical calorie''. Until 1948, the latter was defined as 4.1833 international joules; the current standard of 4.184 J was chosen to have the new thermochemical calorie represent the same quantity of energy as before. The calorie was first defined specifically to measure energy in the form of heat, especially in experimental calorimetry.


In a nutritional context, the kilojoule (kJ) is the SI unit of food energy, although the ''calorie'' is commonly used. The word ''calorie'' is commonly used with the number of kilocalories (kcal) of nutritional energy measured. In the United States, most nutritionists prefer the unit kilocalorie to the unit kilojoules, whereas most physiologists prefer to use kilojoules. In the majority of other countries, nutritionists prefer the kilojoule to the kilocalorie. US food labelling laws require the use of kilocalories (under the name of "Calories"); kilojoules are permitted to be included on food labels alongside kilocalories, but most food labels do not do so. In Australia, kilojoules are officially preferred over kilocalories, but kilocalories retain some degree of popular use. Australian and New Zealand food labelling laws require the use of kilojoules; kilocalories are allowed to be included on labels in addition to kilojoules, but are not required. EU food labelling laws require both kilojoules and kilocalories on all nutritional labels, with the kilojoules listed first. To facilitate comparison, specific energy or energy density figures are often quoted as "calories per serving" or "kcal per 100 g". A nutritional requirement or consumption is often expressed in calories or kilocalories per day. Food nutrients as fat (lipids) contains 9 kilocalories per gram (kcal/g), while carbohydrate (sugar) or protein contains approximately 4 kcal/g. Alcohol in food contains 7 kcal/g. Food nutrients are also often quoted "per 100 g".


In other scientific contexts, the term ''calorie'' almost always refers to the small calorie. Even though it is not an SI unit, it is still used in chemistry. For example, the energy released in a chemical reaction per mole of reagent is occasionally expressed in kilocalories per mole. Typically, this use was largely due to the ease with which it could be calculated in laboratory reactions, especially in aqueous solution: a volume of reagent dissolved in water forming a solution, with concentration expressed in moles per litre (1 litre weighing 1 kilogram), will induce a temperature change in degrees Celsius in the total volume of water solvent, and these quantities (volume, molar concentration and temperature change) can then be used to calculate energy per mole. It is also occasionally used to specify energy quantities that relate to reaction energy, such as enthalpy of formation and the size of activation barriers. However, its use is being superseded by the SI unit, the joule, and multiples thereof such as the kilojoule.

Measurement of energy content of food

In the past, a bomb calorimeter was used to determine the energy content of food by burning a sample and measuring a temperature change in the surrounding water. Today, this method is not commonly used in the United States and has been replaced by calculating the energy content indirectly from adding up the energy provided by energy-containing nutrients of food (such as protein, carbohydrates, and fats), the Modified Atwater system. The fibre content is also subtracted to account for the fact that fibre is not digested by the body.

See also

*Basal metabolic rate *Caloric theory *Conversion of units of energy *Empty calorie *Food energy *Nutrition facts label


{{reflist|30em|refs= {{cite journal|last=Rossini|first=Fredrick| title=Excursion in Chemical Thermodynamics, from the Past into the Future|journal=Pure and Applied Chemistry|year=1964|volume=8|issue=2|page=107|doi=10.1351/pac196408020095 |url=http://iupac.org/publications/pac/8/2/0095/|access-date=21 January 2013|author-link=Frederick Rossini|quote=both the IT calorie and the thermochemical calorie are completely independent of the heat capacity of water.|doi-access=free {{cite web | title=Definition of Calorie | website=Merriam-Webster | date=August 1, 2017 | url=https://www.merriam-webster.com/dictionary/calorie | access-date=September 4, 2017 {{cite book|title=Compendium of Analytical Nomenclature|year=1997|author=International Union of Pure and Applied Chemistry (IUPAC)|author-link=International Union of Pure and Applied Chemistry|isbn=0-86542-615-5|chapter-url=http://iupac.org/publications/analytical_compendium/Cha01sec6.pdf|edition=3|access-date=31 August 2013|chapter=1.6 Conversion tables for units|url-access=registration|url=https://archive.org/details/compendiumofanal0000unse Zvi Rappoport ed. (2007)
"The Chemistry of Peroxides"
Volume 2 page 12.
{{cite web |url=http://www.fao.org/docrep/meeting/009/ae906e/ae906e17.htm |author=FAO |author-link=FAO |title=The adoption of joules as units of energy |year=1971 {{cite journal|last=Hargrove|first=James L|title=Does the history of food energy units suggest a solution to "Calorie confusion"? |journal=Nutrition Journal |year=2007|volume=6|issue=44|pages=44|doi=10.1186/1475-2891-6-44 |pmid=18086303|pmc=2238749 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. {{cite web |url=https://dictionary.cambridge.org/us/dictionary/english/calorie |title=Cambridge Dictionary: calorie |access-date=November 9, 2019 {{cite web |url=https://www.oxfordlearnersdictionaries.com/us/definition/american_english/calorie |title=Definition of calorie noun from the Oxford Advanced American Dictionary |access-date=November 9, 2019 Category:Units of energy Category:Heat transfer Category:Non-SI metric units