The paper presents a method of calculation of a transient electromagnetic field in ferromagnetic materials with real characteristics of magnetisation taken into account. Spatial-temporal distributions of the electromagnetic field have been calculated as a result of the solution of the nonlinear partial differential equation for induction B, in space ??(x, t), using the difference method. The paper presents the conditions of stability, convergence and the minimum error of approximation for the differential diagram which is assumed to approximate to a differential equation. The elaborated method includes the problems of calculating losses and resistance for eddy currents in the ferromagnetic material in transient states for nonlinear media and boundary conditions arbitrarily changing in time.