[1] In terms of SI base units, one joule corresponds to one kilogram-square metre per square second (1 J = 1 kg⋅m2⋅s−2).

It is also the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second.

As with every SI unit named for a person, its symbol starts with an upper case letter (J), but when written in full, it follows the rules for capitalisation of a common noun; i.e., joule becomes capitalised at the beginning of a sentence and in titles but is otherwise in lower case.

The naming of the unit in honour of James Prescott Joule (1818–1889), at the time retired and aged 63, followed the recommendation of Siemens: Such a heat unit, if found acceptable, might with great propriety, I think, be called the Joule, after the man who has done so much to develop the dynamical theory of heat.

[8]At the second International Electrical Congress, on 31 August 1889, the joule was officially adopted alongside the watt and the quadrant (later renamed to henry).

[10] In 1935, the International Electrotechnical Commission (as the successor organisation of the International Electrical Congress) adopted the "Giorgi system", which by virtue of assuming a defined value for the magnetic constant also implied a redefinition of the joule.

The joule was explicitly intended as the unit of energy to be used in both electromagnetic and mechanical contexts.

[11] The ratification of the definition at the ninth General Conference on Weights and Measures, in 1948, added the specification that the joule was also to be preferred as the unit of heat in the context of calorimetry, thereby officially deprecating the use of the calorie.

[33] The use of newton-metres for torque but joules for energy is helpful to avoid misunderstandings and miscommunication.

where E is energy, τ is (the vector magnitude of) torque, and θ is the angle swept (in radians).

[citation needed] A watt-second (symbol W s or W⋅s) is a derived unit of energy equivalent to the joule.