In this paper, we consider Ca insertion in the classical NASICON structures of Ca y M 2 (PO 4 ) 3 with M = Ti and Zr adapted from experimental XRD data. As already mentioned above, they have been chosen because they have the same number of valence electrons but differ in size with the Zr ion being about 15 % larger than the Ti ion. The considered phases show a rhombohedral structure. As the acronym NASICON, sodium (Na) super ionic conductor, already suggests, these materials have originally been used as solid electrolytes for sodium ion batteries. However, they have recently also become of interest as possible electrode materials for calcium-ion batteries. Given the oxidation states of the considered elements, the Ca-NASICON structures will in general be able to remain electrostatically neutral for an occupation of 0.5 ≤ y ≤ 2.5 calcium per formula unit.