The Joule [ dʒu ː l] is the SI derived unit of sizes energy, work and heat. Named after James Prescott Joule, this unit is now used for all forms of thermal, mechanical and electrical energy.

As each derived mechanical unit, the joule by the base units kg, m and s are expressed:


A Joule is equal to the energy that is required to:

  • To move the point of attack a force of one newton against the same by one meter ( 1 newton meters) - about a body with mass 0.102 kg ( which is about a chocolate bar ) to raise by one meter - or
  • One second the power of one watt - which is about the power of the human heart (see: magnitude (power) ) - to be provided (1 watt-second ) or
  • To flow an electric current of one ampere of one volt for the duration of one second with an electrical voltage ( 1 volt ampere second) or
  • One gram of water to be heated to about 0,239 K.

In nutritional labels kilojoules are ( kJ ) - based on 100 g, 100 ml or 1 serving - for the indication of the physiological caloric value used.


The pronunciation of the word " joule " is inconsistent: In German-speaking pronunciation [ dʒu ː l] is common James Prescott Joule pronounced his name probably like this.


At the first International Electricity Congress on 20 and 21 September 1881 in Paris not only the CGS system was made ​​compulsory, but in addition to this " absolute system " and a " practical system " consisting of ohms, amps, volts, Coulomb and farads, which for practical use in electrical manageable sizes provides as directly derived from the CGS system units. Both systems but differ only by factors of powers of ten.

In his inaugural speech as President of the British Association for the Advancement of Science Wilhelm Siemens has also proposed the joule as the unit of heat to supplement the practical system in addition to Weber, W and Gauss on 23 August 1882. It should be derived from the other electromagnetic units than the amount of heat which produces a current of 1 ampere at a resistance of 1 ohm within 1 second, and thus defined equivalent to 107 erg, units of energy in CGS system.

" The unit of heat Has hitherto been taken variously as the heat required to raise a pound of water at the freezing -point through 1 ° Fahrenheit or Centigrade, or, again, the heat Necessary to raise a kilogram of water 1 ° Centigrade. The inconvenience of a unit so Entirely arbitrary Sufficiently is apparent to justify the Introducion of one based on the electro - magnetic system, viz. the heat generated in one second by the current of an ampere flowing through the resistance of an ohm. In absolute measure its value is 107 C.G.S. units, and, Assuming Joule 's equivalent as 42,000,000, it is the heat Necessary to raise 0238 grammes of water 1 ° Centigrade, or, Approximately, the ⅟ ₁ ₀ ₀ ₀ th part of the arbitrary unit of a pound of water raised 1 ° Fahrenheit and the ⅟ ₄ ₀ ₀ ₀ th of the kilogram of water raised 1 ° Centigrade. Such a heat unit, if found acceptable, with great propriety might also, I think, be called the Joule, after the man who has done so much to develop the dynamical theory of heat. "

" The unit of heat has been defined in various ways, for example as the heat needed to heat one pound of water from the freezing point by 1 ° Fahrenheit or Celsius, or even as the heat that is needed to differently by one kilogram of water heating 1 ° Celsius. The inadequacy of such a completely random unit is obvious enough to justify the introduction of a system based on the electromagnetic unit, namely defined as the heat that is generated when one ampere flows through a resistance of 1 ohm. This value is in the CGS system, the measure is 107 and if one assumes Joules mechanical equivalent of heat with 42,000,000, equal to the heat needed to warm 0.238 gram of water by 1 ° Celsius, or about the 1/1000ste part the randomly specified unit of one pound of water, which is heated by 1 ° Fahrenheit and about the 1/4000ste part of one kilogram of water, which is heated by 1 ° Celsius. If such a unit of heat is generally accepted, it should be called my opinion " Joule ", because this man has done so much for the development of the dynamic theory of heat. "

The second International Electricity Congress in Paris, the Joule finally decided on 31 August 1889 as a unit in the proposed form, next to the Watts and Henry ( then known as quadrant). Joule was thus defines both the CGS - based units and via resistance, amps, although in turn can be derived from the base CGS units, but were still specified with direct measurement rules.

The fourth International Electricity Congress in August 1893 in Chicago has therefore also introduced the international joules, which is derived from the specifications of the international ohm and international ampere, which were also slightly changed compared to the old resistance, amps. In contrast, the absolute joules or joules theory has described the true value based on the CGS system.

In June 1935, the International Electrotechnical Commission, the successor to the International Electricity Congress, the adoption of the Giorgi system has decided an FMD system, refer to the existing "practice units " without the powers of ten as conversion factors a natural place by the magnetic field constant is appropriately defined. Also, the Joule has further had its place here.

In the train of the Metre Convention, the International Committee has defined the joule as the work of Weights and Measures in 1946, which will be provided by the fact that the point at which 1 MKS unit of force (at that time not yet officially Newton named) attaches, is moved by 1 meter in their direction. Expressly Joule was also provided as a unit of energy and for use in the electromagnetic context.

The General Conference on Weights and Measures has this definition ratified at its 9th meeting in 1948 and, in particular also requires that the joule is used in the calorimetry whenever possible instead of calorie. Since 1960, the Joule finally is part of the International System of Units (SI).

In the EU, the EC Directive 71/354/EEC has declared October 18, 1971, the joule as the other SI units for " finally admitted ". Within five years, the EU Member States had to introduce the compulsion to use; the use of the competing Calorie was to prohibit by the end of 1977. The fate of the Erg was initially left open; its use was eventually to prohibit until the end of 1979.

Directive 80/181/EEC of 20 December 1979 (in force since October 1981), the SI confirmed with the Joule, but the Member States the possibility of other units temporarily permitted again, as long as they are only used in addition and the SI unit is highlighted. The deadline was initially set at the end of 1989 and was later extended until the end of 1999 and then to the end of 2009. Since 1 January 2010, the additional indication of calories in addition to Joule in the EU is now allowed without time limitation.

For nutrition labeling both an indication in kJ and kcal is required in the EU. The use of the prefix kilo - is obligatory for nutritional labeling in the EU.


A conversion table for common units of energy can be found in the article energy.

The billing units for energy are in use: