The name calorie is used for two units of energy:
- the small calorie or gram calorie (symbol: cal) which is the approximate amount of energy needed to raise the temperature of one gram of water by one degree Celsius;
- large calorie, kilogram calorie, dietary calorie, nutritionist's calorie or food calorie (symbol: Cal, equiv: kcal), which is the amount of energy needed to raise the temperature of one kilogram of water by one degree Celsius; thus it is equal to 1000 small calories.
Although these units are part of the metric system, they now have been superseded in the International System of Units by the joule. One large calorie is equal to 4184 joules; therefore, one small calorie is 4.184 joules and one large calorie is 4.184 kilojoules. The factors used to convert calories to joules are numerically equivalent to expressions of the specific heat capacity of water in joules per gram or per kilogram. The 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 in the US, UK and some other Western countries. The small calorie is also often used in chemistry as the method of measurement is fairly straightforward in most reactions, though the amounts involved are typically expressed in thousands as kcal, an equivalent unit to the large calorie.
Precise definitions 
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, and is difficult to measure precisely. Accordingly, there have been several definitions of the calorie that attempt to make the definition more precise.
The pressure is usually taken to be the standard atmospheric pressure (101.325 kPa). The temperature increase is often stated to be one kelvin, which by current definitions is exactly equal to an increment of one degree Celsius.
|Thermochemical calorie||calth||≡ 4.184 J|| the amount of energy equal to exactly 4.184 joules |
|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. 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.|
|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. [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).|
Note 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), 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.
The two definitions most common in older literature appear to be the 15 °C calorie and the thermochemical calorie.
The calorie was first defined specifically to measure energy in the form of heat, especially in experimental calorimetry.
In nutritional contexts, the kilojoule (kJ) is the SI unit of food energy. However, calorie and kilocalorie are still in common use.
In these contexts, confusingly, the word "calorie" and "kilocalorie" refer to equivalent units (the former to the large calorie and the latter to 1000 small calories). Sometimes, in an attempt to avoid confusion, the large calorie is written as "Calorie" (with a capital "C"). This convention is not always followed, and not explained to the average person clearly.
These quantities are often used for the total amount of food energy (e.g., in a meal) and for the specific energy, namely amount of energy per unit of mass (e.g. "calories per gram", "calories per serving"). Nutritional requirements or intakes are often expressed in calories per day.
In 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 kilocalorie per mole. This use is largely due to the ease with which it can 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 1kg), 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 kcal/mol. 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 joule unit.
See also 
- Empty calorie
- Basal metabolic rate
- Food energy
- Nutrition facts label
- Conversion of units of energy
- Merriam-Webster's Online Dictionary Def 1a http://www.merriam-webster.com/dictionary/calorie
- Calorie Confusion: The History of the Calorie
- 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.
- FAO (1971). "The adoption of joules as units of energy". "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)."
- 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."
- "Prospects improve for food energy labelling using SI units". Metric Views. UK Metric Association. 24 February 2012. Retrieved 17 April 2013.
- Chemistry Daily.
- Zvi Rappoport ed. (2007), "The Chemistry of Peroxides", Volume 2 page 12.