Abstract The thermodynamic stabilities of alkaline earth (Ca, Sr and Ba), and rare earth (La, Ce and Yb), filled CoSb 3 skutterudites have been studied using a plane-wave density functional method. By combining the formation energy of inserting an impurity into the intrinsic void of CoSb 3 and that of secondary phases ISb 2 and CoSb 2, it is found that the filling fraction limit (FFL) or the maximum filling fraction of an impurity I corresponds to the minimum formation energy for a mixed chemical reaction route that results in the formation of filled skutterudite I y Co 4Sb 12 at the maximum filling as well as the formation of secondary phases. Theoretically estimated FFLs of various impurities in the voids of CoSb 3 are in good agreement with the reported experimental data. A schematic phase diagram for filled CoSb 3 is given. Discussion on the effect of the ionic radius of a filler and the content of Sb on FFL is presented.