In this computational study we have addressed the Ca mobility in NASICON battery materials from first principles. Due to the complex structure of the NASICON material, the determination of the Ca migration paths is computationally rather demanding, limiting the number of systems that can be numerically studied. In particular, we considered the Ca mobility in Ti and Zr-based NASICON materials. These two metals have the same number of valence electrons but different sizes, the Zr ion is about 14 % larger than the Ti ion which makes the crystal structure more open. According to our calculations, the triangular faces of the so-called Na2 octahedra correspond to the bottleneck along the diffusion path with the area of the faces of the TiO 6 octahedron being 20% smaller than those of the ZrO 6 octahedron. In fact, we find that the Zr barrier for Ca diffusion is 23% smaller than the Ti barrier, confirming the notion that the size of the bottlenecks acts as a good descriptor for the ion mobility. This suggests that increasing the size of the bottlenecks, for example by deliberately doping the NASICON material with larger metal ions, provides a route for increasing the ion mobility in these structures.