[en] The invention relates to apparatus for measuring thickness and mass per unit area of sheet material taking into account the effect of changes in composition in nominal values as such changes affect the material's x-ray transmission coefficient etc. If the density of the sheet material is known the thickness can be calculated from the mass per unit area. The X-ray gauge described measures these parameters of moving sheet material (rolled steels, plastic, etc) as it passes a gauging station by measuring the backscatter and attenuation of an X-ray beam. It uses two detectors; the first, positioned on the opposite side of the sheet from the radiation source measures the transmitted radiation, the second is at an angle to measure the scattered beam intensity. From these intensities a thickness measurement can be computed automatically. (U.K.)