A comprehensive model has been developed for calculating electrical conductivities of aqueous or mixed-solvent electrolyte systems ranging from dilute solutions to fused salts. The model consists of a correlation for calculating ionic conductivities at infinite dilution as a function of solvent composition and a method for predicting the effect of finite electrolyte concentration. The effect of electrolyte concentration is calculated using an extended form of the mean-spherical-approximation (MSA) theory coupled with a mixing rule for predicting the conductivities of multicomponent systems on the basis of the conductivities of constituent binary cation-anion subsystems. The MSA theory has been extended to very concentrated and mixed-solvent systems by introducing effective ionic radii that take into account various interactions between ions, solvent molecules, and ion pairs. The model has been coupled with thermodynamic equilibrium computations to provide the necessary concentrations of individual ions in complex, multicomponent systems. The model accurately reproduces experimental conductivity data over wide ranges of composition with respect to both solvents and electrolytes. In particular, the model is shown to be accurate for aqueous acids (e.g., H2SO4, HNO3, and H3PO4) up to the pure acid limit, various nitrates ranging from dilute solutions to fused salts, salts in water + alcohol mixtures, and LiPF6 solutions in propylene and diethyl carbonate.