An equation for the rate of an electrochemical reaction; it describes the current density at an electrode in terms of the overpotential. The Butler–Volmer equation is given by:

where j_a and j_c are the individual cathode and anode currents respectively, and j_e is the equilibrium current, called the exchange current density. By definition

where j_ce is the equilibrium cathode current and j_ae is the equilibrium anode current. F is the Faraday constant, η‎ is the overpotential, R is the gas constant, T is the thermodynamic temperature, and α‎ is a quantity called the transfer coefficient.

The Butler–Volmer equation was derived by John Alfred Valentine Butler (1899–1977) in 1924 and by Max Volmer (1885–1965) in 1930.