Abstract: The thickness of a hot metal plate is measured during rolling by determining the absorption by the plate of gamma radiation transmitted through the plate, the gamma radiation having at least one characteristic energy. A measurement value of the intensity of the radiation after passage of the plate is provided by digitally counting pulses generated by a radiation detector. Essentially only pulses which belong to the photopeak of said at least one characteristic energy in the pulse spectrum obtained from the detector are counted. The counted number of pulses is corrected for dead time losses by using a live time counter.Corrected mass absorption coefficients corresponding to partial thickness ranges within the total thickness range are calculated from the measured intensity of radiation passed through calibrating plates of differing thicknesses.