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One 24-ounce energy drink has 330 calories, more than a fast-food cheeseburger, and the equivalent of 18 single-serving packets of sugar.

Calories are units of energy. Various definitions exist but fall into two broad categories, the first, the small calorie, or gram calorie (symbol: cal), is defined as the approximate amount of energy needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere.[1] The second is the large calorie or kilogram calorie (symbol: Cal), also known as the food calorie and similar names,[2] is defined in terms of the kilogram rather than the gram. It is equal to 1000 small calories or 1 kilocalorie (symbol: kcal).[1]

Although these units relate to the metric system, all of them have been considered obsolete in science since the adoption of the SI system,[3] the unit of energy in the International System of Units is the joule. One small calorie is approximately 4.2 joules (so one large calorie is about 4.2 kilojoules). The factor used to convert calories to joules at a given temperature is numerically equivalent to the specific heat capacity of water expressed in joules per kelvin per gram or per kilogram, the precise conversion factor depends on the definition adopted.

In spite of its non-official status, the large calorie is still widely used as a unit of food energy, the small calorie is also often used for measurements in chemistry, although the amounts involved are typically recorded in kilocalories.

The calorie was first defined by Nicolas Clément in 1824 as a unit of heat energy[3] and entered French and English dictionaries between 1841 and 1867. The word comes from Latin calor, meaning 'heat', the large calorie was introduced to the American public by Wilbur Olin Atwater in 1887.[3]


The energy needed to increase the temperature of a given mass of water by 1 °C depends on the atmospheric pressure and the starting temperature. Accordingly, several different precise definitions of the calorie have been used.

The pressure is usually taken to be the standard atmospheric pressure (101.325 kPa). The temperature increase can be expressed as one kelvin, which means the same as an increment of one degree Celsius.

Name Symbol Conversions Notes
Thermochemical calorie calth 4.184 J

≈ 0.003964 BTU ≈ 1.162×10−6 kWh ≈ 2.611×1019 eV

the amount of energy equal to exactly 4.184 joules [4][a][6][7]
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 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 to 15.5 °C at standard atmospheric pressure. Experimental values of this calorie ranged from 4.1852 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 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

1100 of the amount of energy required to warm one gram of air-free water from 0 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

1860 international watt hours = 18043 international joules exactly.[note 1]
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]
  1. ^ The 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[8]), 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 and 4.18674 J, respectively.

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.[6]

The calorie was first defined specifically to measure energy in the form of heat, especially in experimental calorimetry.[9]


In a nutritional context, the kilojoule (kJ) is the SI unit of food energy, although the kilocalorie is still in common use,[10] the word calorie is popularly used with the number of kilocalories of nutritional energy measured. As if to avoid confusion, it is sometimes written Calorie (with a capital "C") in an attempt to make the distinction, although this is not widely understood. Capitalization contravenes the rule that the initial letter of a unit name or its derivative shall be lower case in English.[11]

To facilitate comparison, specific energy or energy density figures are often quoted as "calories per serving" or "kilocalories per 100 g". A nutritional requirement or consumption is often expressed in calories per day. One gram of fat in food contains nine kilocalories, while a gram of either a carbohydrate or a protein contains approximately four kilocalories.[12] Alcohol in a food contains seven kilocalories per gram.[13]


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.[14] 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 liter (1 liter weighing 1 kg), 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.[15] However, its use is being superseded by the SI unit, the joule, and multiples thereof such as the kilojoule.

Measurement of calories in food[edit]

In the past a bomb calorimeter was utilised 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 USA and has been succeeded 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 fibre content is also subtracted to account for the fact fibre is not digested by the body.[12]

See also[edit]


  1. ^ "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 has been redefined as 4.1840 J exactly)."[5]


  1. ^ a b "Definition of Calorie". Merriam-Webster. August 1, 2017. Retrieved September 4, 2017. 
  2. ^ Conn, Carole; Len Kravitz. "Remarkable Calorie". University of New Mexico. Retrieved 30 April 2014. 
  3. ^ a b c Hargrove, James L (2007). "Does the history of food energy units suggest a solution to "Calorie confusion"?". Nutrition Journal. 6 (44). PMC 2238749Freely accessible. PMID 18086303. doi:10.1186/1475-2891-6-44. Retrieved 31 August 2013. 
  4. ^ 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.
  5. ^ FAO (1971). "The adoption of joules as units of energy". 
  6. ^ a b Rossini, Fredrick (1964). "Excursion in Chemical Thermodynamics, from the Past into the Future". Pure and Applied Chemistry. 8 (2): 107. doi:10.1351/pac196408020095. Retrieved 21 January 2013. both the IT calorie and the thermochemical calorie are completely independent of the heat capacity of water. 
  7. ^ Lynch, Charles T. (1974). Handbook of Materials Science: General Properties, Volume 1. CRC Press. p. 438. Retrieved 8 March 2014. 
  8. ^ International Union of Pure and Applied Chemistry (IUPAC) (1997). "1.6 Conversion tables for units". Compendium of Analytical Nomenclature (PDF) (3 ed.). ISBN 0-86542-615-5. Retrieved 31 August 2013. 
  9. ^ Allain, Rhett (February 23, 2016). "Calculating Calories by Burning Gummy Bears to Death". Scientific American. Retrieved September 7, 2017. 
  10. ^ "Prospects improve for food energy labelling using SI units". Metric Views. UK Metric Association. 24 February 2012. Retrieved 17 April 2013. 
  11. ^ "SI Conventions". National Physical Laboratory. Retrieved 8 February 2016. 
  12. ^ a b "How Do Food Manufacturers Calculate the Calorie Count of Packaged Foods?". Scientific American. Retrieved 2017-09-08. 
  13. ^ "Calories - Fat, Protein, Carbohydrates, Alcohol. Calories per gram". 
  14. ^ Zvi Rappoport ed. (2007), "The Chemistry of Peroxides", Volume 2 page 12.
  15. ^ Bhagavan, N. V. (2002). Medical Biochemistry. Academic Press. pp. 76–77. ISBN 9780120954407. Retrieved 5 September 2017.