The equation for the dynamic plate thickness is derived as a function of standoff distance Z(0), jet tip velocity nu(j0), cutoff velocity nu(jc), plate velocities nu(PI) and NATO angle of the ERA sandwich. The dynamic thickness is presented as functions of the different parameters for the front plate - flying against the shaped charge jet - and the rear plate - flying with the shaped charge jet. But the dynamic thickness is only one of the reduction factors of ERA sandwiches.