Ion size is an important parameter in interpreting the electrochemical properties of the electrolyte materials. There is difficulty in selecting a representative value for ionic radius when the ion has a shape far from spherical. Since the van der Waals volume becomes a good parameter for ion size, it was calculated by numerical integral based on a simple overlapped spherical model using the optimized structures by ab initio molecular orbital calculation. We have examined the extent of errors affected by the coordinates, and given the van der Waals radii of each atom, and the integral grid size, and give the van der Waals volumes of various ions applied for the electrolyte materials of lithium batteries and double-layer capacitors.