Methods are outlined for computer calculation of first- and second-order contributions to gamma-ray dose rates in two-legged rectangular concrete ducts of personnel shelters. Four computer programs, based on values of differential dose albedo derived from Monte Carlo calculations, are used to calculate corner-lip inscattering, backscattering from surfaces visible to both source and detector, and combinations of these two scattering events. The first program computes the dose contribution from corner-lip inscatter and from combinations of corner-lip penetration and wall backscatter. The second calculates first-order backscatter from the basic scattering areas in the corner of the duct. The third computes the second-order backscatter contribution from two successive backscatters from the duct walls. The final program calculates the contribution due to combinations of one wall backscatter and one corner-lip inscatter. The results of the calculations are compared with experimental results obtained by several investigators. The experimental studies used 11 × 11-in., 3 × 3-ft, and 6 × 6-ft ducts; the gamma-ray sources were, variously, 198Au, 137Cs, 60Co, and 24Na. Since the difference between calculated and measured values of gamma-ray dose in the second leg is generally less than 30%, results obtained by calculation can be used to predict the attenuation factors of two-legged concrete ducts and entranceways leading into personnel shelters with an expected error of less than 30%.