An x-ray diffraction method for determining thicknesses of thin films grown on single-crystal substrates is presented. The equations, based on the kinematical theory of x-ray diffraction and the mosaic crystal model, were developed. The thickness of the thin film was computed from the absorption of the integrated diffracted x-ray intensity from the single-crystal substrate. Since the diffracted intensity from the film is not required, the film does not have to be single crystal in nature. Thus, thicknesses of less ordered, polycrystalline, or even amorphous films can be measured with high precision by this technique.