Energy-dependent photoemission was measured to investigate the validity of the analysis of line shape in determining the absolute coverage of atomically flat, metallic thin films. The surface states of two Ag/Au(1 1 1) thin films with carefully controlled coverage of Ag were measured and analysed. Our results confirm that line-shape analysis is a valid procedure; the absolute error associated with this technique is within 0.1 ML, which makes the technique advantageous over other techniques to determine the film coverage. The experimental procedure in our work provides a routine to determine an appropriate photon energy for use in line-shape analysis. Our results indicate that the widely accessible He Iα line is a suitable excitation source to utilize line-shape analysis for confined states in a Ag film.