An approximate equation is derived for the steady-state limiting current of mediated bioelectrocatalysis as a function of the bulk mediator concentration in the case where enzyme and mediator are in the soluble state and the substrate concentration is sufficiently higher than the Michaelis constant of the substrate. The error of the approximate equation has been evaluated by digital simulation and an empirical equation to correct the error has been proposed to describe the steady-state current more precisely. These equations have been applied successfully to typical mediated bioelectrocatalytic systems to evaluate the catalytic constant and the Michaelis constant of the mediator.