Ionospheric transmission and reflection coefficients for vertical incidence are determined by means of a simple and rapid numerical method based on thin-film optics. By this method the ionosphere is divided into a large number of thin layers, all partial reflections within a given layer are summed, and the process is then extended layer by layer to include the entire ionospheric region. The iterative procedure in its simplest form is started high in the ionosphere and extended downward to terminate at the ionospheric base, but by suitable modification it may be started low in the ionosphere and extended upward to yield in a single ‘run’ the contribution of each portion of the electron density profile to the over-all reflection and transmission coefficients. The method is free of the tendency to numerical swamping and is valid as long as the wave polarization remains constant, which is true in the D region in most of the VLF range. Comparison with full-wave analysis shows that the method breaks down at higher frequencies when weak coupling processes between the magnetoionic modes become relatively important owing to varying wave polarizations.