By generalization of a method due to Politzer et al. [J. Chem. Phys. 79, 3859 (1983)], it is demonstrated how the absolute hardness of an electronic system can be determined from the electrostatic potential, as a function of position of the system and its positive and negative ions. It is shown that to good accuracy the hardness is one-half the electrostatic potential at the covalent radius due to the Fukui function.