At small values of current density and in the absence of significant forced convection, the simultaneous mass transfer of reagent and of product is controlled by free convection. A practical mass transfer equation is derived applicable to laminar as well as to turbulent flow. The function considers the particular conditions of electrodes with and without gas evolution and differs in application to reagent and to product. In addition to the more accurate and complex equation, a simpler relationship suitable for rough estimates is suggested.