[en] We present a new method to extract momentum distributions from time-dependent wavepacket calculations. In contrast to established Fourier transformation of the spatial wavepacket at a fixed time, the proposed 'virtual detector' method examines the time dependence of the wavepacket at a fixed position. In first applications to the ionization of model atoms and the dissociation of H₂⁺, we find a significant reduction of computing time and are able to extract reliable fragment momentum distributions by using a comparatively small spatial numerical grid for the time-dependent wavefunction