Ionic conductivity is a fundamental property of ionic liquids with its origin and exact nature under debate. Using a specially selected system of pyrrolidinium cations (PYR1x , x =3,4) and sulfonylimide anions (FSI, TFSI, BETI, and IM14)-based ionic liquids we observe a simple and accurate connection between ab initio computed cation and anion volumes and measured molar ionic conductivities.