A method, system, and software for calculating the true coincidence summing effects for radionuclide spectroscopy analysis. A set of equations which can be implemented with a stored computer program performs calculations to correct the spectroscopic analysis data for gamma-gamma coincidence events, gamma-X-ray coincidence events, as well as gamma-annihilation photon coincidence events. The net gamma-ray, KX-ray and annihilation photon summing-out probabilities for the analytic gamma-ray is the total summing-out probability from all gamma-ray summing-out chains subtracted by the total summing-out probability from all gamma-ray sub-cascade chains involving the analytic gamma-ray. A total efficiency calculation is performed to eliminate the need for using radioactive sources to create summing specific calibration measurements and for increased accuracy.