A new method is introduced for the calculation of transmission probabilities in resonant tunneling. Probability fluxes at the entrance and at the exit scattering channels are integrated as functions of time, and a weighted average of them 'on the fly' provides the infinite-time limit of the transmission probability, The convergence is obtained within the direct-scattering timescale, regardless of time-delays due to population of long-lived resonances. Therefore, the propagation time may be shortened by orders of magnitude. The method is derived and demonstrated for resonant tunneling through different double-barrier structures, in one dimension and in three dimensions. (C) 2002 Published by Elsevier Science B.V.