For the meaning of any metric prefix, go here.

Convert between joules or megajoules and other major units of energy.

Convert between joules per second and other major units of power.

The unit of energy in SI. Symbol, J. The work done when the point of application of a force of 1 newton is displaced 1 meter in the direction of the force. One watt-second is equal to 1 joule.

The joule’s dimensions are L²MT⁻². It is force × length (newton × meter, or in terms of base units only:)

a fraction, meter squared times kilogram over seconds squared.

portrait of Joule

The joule is named for James Prescott Joule (1818 – 1889), who in 1845 was the first to measure the equivalence of work and heat, by using falling weights to rotate paddles in water and measuring the rise in temperature of the water.

The joule was adopted in 1889 by the International Electrical Congress in Paris. When the CGPM first defined SI, in 1960, it included the joule as one of the derived units.



Fifteenth Report— Bath, 1888.

It was also agreed to adopt the name “Joule” for 10⁷ C.G.S. units of work. Thus a Joule is equal to 10⁷ ergs. It is the work done in one second by the power of one Watt, or again the work done when a current of one Ampere flows for one second between two points between which the difference of potential is one Volt, and hence a power of one Watt is one Joule per second.

F. E. Smith, editor.
Reports of the Committee on Electrical Standards Appointed by the British Association for the Advancement of Science.
Cambridge (UK): Cambridge University Press, 1913.
Page 342.


The International Electrical Conference at Paris

[excerpts from the article]

Paris, France, Aug. 27, 1889.— … M. Guillaume also presented some considerations on uniformity of notation. He recalled the decisions taken by the International Committee on Weights and Measures for the metric units, and proposed to adopt an analogous system for the mechanical and electrical units. The system consists in designating each of the units dyne, erg, barie, watt, volt, ampere, farad, coulomb and joule by two initials in Roman characters; the ohm would be designated by ω; the usual multiples and submultiples would be the following: mega M, kilo k, milli m and micro µ.

Paris, Aug. 29, 1889.— The Committee appointed for the purpose brought in the resolutions to their section on the adoption of the joule and the watt, which are given in the report of Saturday's proceedings, to be found below. A discussion followed their reading.

Paris, Aug. 30, 1889.— The meeting was opened by the section approving the conclusions of the committee on units relative to the adoption of the watt and joule, and given in the last day's proceedings.

Paris, Aug. 31, 1889.— The President then put to a vote the various recommendations adopted by the different sections as follows:

The practical unit of work is the Joule. It is equal to 107 C. G. S. units of work; it is the energy expended during one second by one ampere through a resistance of one ohm.

The Electrical World, vol. 14, no. 3, pages 202 -204 (September 21, 1889).
L'unité pratique de travail est le joule. Il est égal à 107 unités C.G.S. de travail. C'est l'énergie dépensée pendant une seconde par un ampère dans un ohm.


Report of the Action of the International Electrical Congress held in Chicago, August 1893, in the Matter of Units of Electrical Measure.

Meetings of the Chamber [of Delegates] continued during six days, at the end of which its members unanimously agreed in the adoption of the following resolution :—

Resolved, That the several Governments represented by the delegates of this International Congress of Electricians be, and they are hereby, recommended to formally adopt as legal units of electrical measure the following:

As a unit of work, the joule, which is equal to 107 units of work in the C.G.S. system, and which is represented sufficiently well for practical use by the energy expended in one second by an international ampere in an international ohm.

From the report by the U.S. delegates (Rowland, Mendenhall, et al) to the U.S. Secretary of State.


Twenty-Third Report — Liverpool, 1896.

Proposition I. — For many purposes heat is most conveniently measured in units of energy, and the theoretical C.G.S. unit of heat is 1 erg. The name Joule has been given by the Electrical Standards Committee to 107 ergs.

For many practical purposes heat will continue to be measured in terms of the heat required to raise a measured mass of water through a definite range of temperature.

If the mass of water be 1 gramme, and the range of temperature 1° C. of the hydrogen thermometer from 9.5° C. to 10.5° C. of the scale of that thermometer, then, according to the best of the existing determinations, the amount of heat required is 4.2 Joules.

It will, therefore, be convenient to fix upon this number of Joules as a secondary unit of heat.

This secondary thermal unit may be called a “Calorie.”

F. E. Smith, editor.
Reports of the Committee on Electrical Standards Appointed by the British Association for the Advancement of Science.
Cambridge (UK): Cambridge University Press, 1913.
Page 543.


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