The mass thickness of thin metal films is an important parameter determining the film structure, however a robust routine method for its determination is still missing. In this work, we have developed a simple optical method for determining the mass thickness of gold films with arbitrary morphology on transparent substrates in the range 1–20 nm. The method is based on film spectrophotometry in the far-UV range when the light attenuation is due to interaction with core electrons and thus the influence of the film morphology is negligible. The calibration was performed at a wavelength of 200 nm for gold films of known thickness and a good agreement with the Beer–Lambert law was obtained. The feasibility of the method was demonstrated with gold films of various morphologies produced on quartz substrates by pulsed laser deposition.