The values of buildup factors for a specific energy above K edges and penetration distance vary smoothly with respect to atomic number. An interpolation of buildup factors for an arbitrary elemental material is examined using geometric-progression (G-P) parameters for an equivalent atomic number. The G-P parameters are data fitted to the proposed American National Standard buildup factor data compilation of 17 elements from beryllium to molybdenum and are calculated by the moments method. The data for iron, molybdenum, tin, lanthanum, gadolinium, tungsten, lead, and uranium, including bremsstrahlung and fluorescence, are calculated by the PALLAS code. Various tests over a wide range of atomic numbers confirm that values of the buildup factors generated by interpolated G-P parameters can reproduce the basic data calculated directly over the full range of energy with an accuracy within a few percent. The values of equivalent atomic number for mixture materials, such as water, air, concrete, and lead glass of 4.36 density, are determined from a ratio of scattering cross section to the total attenuation coefficient. The buildup factors for these materials calculated using the G-P parameters, interpolated by the equivalent atomic number, are in good agreement with the basic data, although a deviation is observed above 3 MeV for the buildup factors for lead glass.