The abbreviation of British thermal unit. A Btu is defined as the amount of heat required to raise the temperature
of one pound of water by one degree Fahrenheit (calorie/pound.Fahrenheit) under a constant pressure of 1 atmosphere.
The Fifth International Conference on the Properties of Steam (London, July 1956) defined the International Table
calorie as 4.1868 J. Therefore the exact conversion factor for the International Table Btu_{IT} is 0.45359237 x 4.1868 x 5/9 = 1.055 055 852 62 kJ. This value is most widely used Btu definition.

British thermal unit based on the 15 °C calorie, Btu_{15} is equivalent to the amount of heat of 1054.72825813056 joules.

Thermochemical Btu (symbol Btu_{th}), based on the definition of the thermochemical calorie (exactly 4.1840 joules) by
the U.S. Bureau of Standards in 1953, is 1054.35026448889 joules.

International Steam Table calorie is defined as 1 cal_{IT} = 4.1868 J exactly. This definition was adopted by the Fifth International
Conference on Properties of Steam (London, July 1956).

15 °C calorie (cal_{15}) is defind as the amount of energy required to warm 1 gram of air-free water from 14.5 °C to 15.5 °C at a constant pressure
of 101.325 kPa (1 atm). 1 calorie15 = 4.1855 joules. This value is uncertain by 0.0005 J, you can see different defintion of 4.1858 listed in some other resouces.

20 °C calorie is defined as the amount of energy required to warm 1 g of air-free water from 19.5 °C to 20.5 °C at a constant pressure of 101.325 kPa (1 atm). This value is about 4.1819 J (NIST).

The unit of energy used in nutrition, also called large calorie, a special name for kilocalorie, which is the amount of heat required to raise the temperature of 1 kg of water 1C. This value is defined as 4184 joules based on thermochemical calorie.

Non-SI unit of energy, which is defined as 4.184 J exactly.

Mean calorie is defined as 1/100 of the amount of energy required to warm 1 gram of air-free water from 0 °C to 100 °C at a constant pressure of 101.325 kPa (1 atm). This value is about 4.190 joules.

The unit used with SI, whose value is obtained experimentally. It is the kinetic energy acquired by electron in passing through a potential difference of 1 volt in vacuum. 1 electronvolt is about 1.602177 x 10^{-19} joules.

The non - SI small unit of energy associalted with CGS system. It is equivalent to the work done when a force of one dyne exerted for a distance of one centimeter, therefore, it is considered to be a special name for one gram
centimeter-squared per second squared (g·cm²/s²), which is equal to 10^{-7} J.
.

The unit of energy and work (symbol ft·lbf) based on foot, pound, second system.It is the work of 1 pound of force done to move an object a distance of 1 foot in the direction of the force.

The unit of energy and work based on foot, pound, second system.It is the work of 1 poundal of force done to move an object a distance of 1 foot in the direction of the force.

The small calorie or gram calorie approximates the energy needed to increase the temperature of 1 gram of water by 1 °C, which is about 4.184 joules.

Unit of work and energy based on metric horsepower per hour (symbol PS.h). 1 metric horsepower is defined as 735.49875 W, therefore, 1 horsepower hour is 2647795.5 joules.

SI unit of work, energy and amount of heat (symbol J). The joule is defined as the work done when the point of application of 1 newton of force moves a distance of 1 metre in the direction of the force.

Non-SI unit of energy (symbol kcal), which is defined as 1000 times of 15°C calorie.

Non-SI unit of energy, which is defined as 1000 times of International Steam Table calorie, i.e. 4186.8 joules.

Non-SI unit of energy, which is defined as 1000 times of thermochemical calorie, i.e. 4184 joules.

The large calorie or kilogram calorie approximates the energy needed to increase the temperature of 1 kg of water by 1 °C. This is about 4.184 kJ, and exactly 1000 small calories.

A unit of work and energy (symbol kW.h). It is the engergy produced under the power of 1 kilowatt for one hour.

A unit of heat energy based on meter-tonne-second system sometimes used by European engineers. The thermie is the amount of energy required to raise the temperature of 1 tonne of water by 1°C. The thermie is equivalent to 1000 calories or 4.1868 megajoules.