Calorie

The calorie is a pre-SI metric unit of energy. It was first defined by Nicolas Clément in 1824 as a unit of heat, entering French and English dictionaries between 1841 and 1867.^[1] In most fields its use is archaic, having been replaced by the SI unit of energy, the joule.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.

Definitions of a calorie fall into two classes:

• The small calorie or gram calorie (symbol: cal)^[2] approximates the energy needed to increase the temperature of 1 gram of water by 1 °C. This is about 4.2 joules.
• The large calorie, kilogram calorie or food calorie (symbol: Cal)^[2] approximates the energy needed to increase the temperature of 1 kilogram of water by 1 °C. This is exactly 1000 small calories or about 4.2 kilojoules.

In scientific contexts, the name "calorie" refers to the gram calorie, and this unit has the symbol cal. Metric prefixes are used with this name and symbol, so that the kilogram calorie is known as the "kilocalorie" and has the symbol kcal. In other contexts, the kilocalorie is often referred to as a Calorie (capital "C"), or just a calorie, and it has to be inferred from the context that the small calorie is not intended. When referring to food energy, calorie refers to the large calorie.

In other words, 1 kcal = 1000 cal = 1 Cal

The conversion factor between calories and joules is numerically equivalent to the specific heat capacity of liquid water (in SI units).

Contents 1 Variations 2 Conversions 3 Notes and references 4 See also

Variations

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	[3]
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.[3]
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	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	1⁄860 international watt hours = 180⁄43 international joules exactly.[4]
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).[3]
IUNS calorie		≡ 4.182 J	This is a ratio adopted by the Committee on Nomenclature of the International Union of Nutritional Sciences.[5]

Conversions

One gram calorie is approximately:

• 4.184 J (joules)
• 0.003964 BTU (British thermal units)
• 1.163×10⁻⁶ kW·h (kilowatt hours)
• 2.611×10¹⁹ eV (electron volts)

One kilogram calorie (food calorie) is approximately:

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

Notes and references

See also

• Basal metabolic rate
• Frigorie
• Food energy
• Nutrition facts label