Abstract: An apparatus and method for calculating gamma radiography variables is provided. The apparatus includes a base with a circular logarithmic time scale displayed thereon, a first disk concentric and rotatably coupled to said base for displaying a circular logarithmic radiation source scale and a circular logarithmic source-to-film distance scale, a second disk displaying a circular nearly linear steel thickness scale and a radial tick mark F, and a third disk displaying a radial baseline mark and a plurality of film type marks. The method disclosed allows calculation of exposure time from known quantities: film type, source-to-film distance, steel thickness, radiation source type and strength.