In this study geometric probability concepts and stereological principles are used to derive an equation for the mean spacing of air voids in hardened concrete. The equation is presented and it is shown that the input paramerers for the equation are the same as those obtained from the standard air-void system analysis of hardened concrete. The equation is valid for all values of paste-to-air ratio. The mean spacing yields a better estimate of the actual spacing of the air voids in hardened concrete than the standard spacing factor.