An elementary charge transfer step can be described by the Butler–Volmer model proposed by John Alfred Valentine Butler and Max Volmer. The reaction rate is given by the Butler-Volmer equation:
In this equation  is the net current density,  is the exchange current density,  is the charge transfer coefficient,  is the number of electrons transferred in the reaction,  is the Faraday constant,  is the molar gas constant,  is the absolute temperature,  is the electrode overpotential,  is the thermodynamic equilibrium reduction potential and  is the observed value of this potential.
The equation yields a negative current density for a reduction reaction (negative overpotential) and a positive current density for an oxidation reaction (positive overpotential). The sign of the current density has no physical meaning and is defined by an international convention.