Seismic earth pressures acting on basement walls and retaining walls are most commonly computed using limit state methods, in which the effects of earthquake shaking are represented by a horizontal body force in an active soil wedge. Limit state methods provide a poor physical representation of the fundamental mechanisms that give rise to seismic earth pressures, which depend on relative wall\u2013soil displacements. Such displacements are a consequence of soil\u2013structure interaction, which, in the absence of a strong inertial component (e.g. from a connected structure), are mainly sensitive to the ratio of wavelength-to-wall height and relative wall-to-soil flexibility. We present a simplified single-frequency procedure for computing seismic earth pressures applied to flexible retaining structures by vertically propagating shear waves. The procedure accounts for the first-order wavelength and wall flexibility effects while simplifying a number of secondary effects in a manner that produces a slightly conservative outcome. Input parameters to the proposed solution are readily attainable for engineering design applications. For typical earth retention systems, earth pressures computed using the proposed procedure are lower than those computed using limit state solutions. Predictions from the proposed solution compare well with results of numerical simulations and centrifuge modeling from literature, whereas limit state procedures either do not provide a physically meaningful solution or produce strongly biased predictions (overprediction of experiments, underprediction of available simulations).