A thorough assessment of the performance of an imaging system includes a measurement of the detective quantum efficiency, DQE(f). One of the terms which is required to calculate DQE(f) is the x-ray fluence (photons/mm2). Diagnostic x-ray systems make use of a wide variety of x-ray spectra, with different kV's, waveforms, and filtration. However, most investigators do not have the equipment to measure the x-ray spectrum directly. The determination of x-ray quantum fluence, which is strongly dependent upon the spectra, is therefore left to approximation. In this paper, a wide variety of x-ray spectra for both mammography (with Mo, Rh and W anodes and 18 - 40 kV) and general diagnostic radiography (W anode, 40 - 140 kV) with different patient (Plexiglas) thicknesses were modeled (using a technique which interpolates measured x-ray spectra), and the quantum fluence was tabulated. Five extensive tables (including the calculated HVL's) are provided to allow investigators to interpolate fluence values appropriate to the x-ray system under study. It is hoped that usage of these tables will prove useful to investigators in their assessment of system performance, and perhaps better consistency between measurements can be achieved.