A new method for the fast estimation of the on-body propagation loss is proposed. The method is applicable for a variety of propagation models but limited by a far-field zone observation point. The technique is based on the equivalent source theorem. Knowing the propagation losses for an infinitesimal current source, propagation losses of real antennas can be obtained with only the knowledge of their equivalent currents over a surface surrounding the antenna. The method can be applied for any kind of excitation signals and offers reduction in simulation time. It has been verified using both monochromatic and Gaussian pulse excitation and good agreement is obtained compared with a full conventional FDTD model with a simple antenna.