The Tafel equation is an equation in electrochemical kinetics relating the rate of an electrochemical reaction to the overpotential.  The Tafel equation was first deduced experimentally and was later shown to have a theoretical justification. The equation is named after Swiss chemist Julius Tafel.
Where an electrochemical reaction occurs in two half reactions on separate electrodes, the Tafel equation is applied to each electrode separately. On a single electrode the Tafel equation can be stated as:
where
* the plus sign under the exponent refers to an anodic reaction, and a minus sign to a cathodic reaction,
* : overpotential, V
*  : "Tafel slope", V
*  : current density, A/m
* : "exchange current density", A/m.
A verification plus further explanation for this equation can be found here. The Tafel equation is an approximation of the Butler-Volmer equation in the case of .  Also, at a given electrode the Tafel equation assumes that the reverse half reaction rate is negligible compared to the forward reaction rate.