A Dynamic Link Library (DLL) was created for calculating true-coincidence correction factors in γ- and X-ray spectrometry applications. Calculations are done by Monte-Carlo method on the basis of the Evaluated Nuclear Structure Data File (ENSDF). In calculations the emission of cascade gammas, as well as γ-X, γ-γ511 and X-X coincidences are appropriately taken into account. A set of measurement geometries implemented includes combinations of coaxial, planar, low and broad energy as well as well-type detectors with point, disk, spherical, cylindrical and Marinelli beaker sources. Together with true-coincidence correction factors full-energy-peak efficiencies are calculated and reported. The DLL was tested by comparing its results with experimental data and results of similar calculations.