A ring current model describing the flow of delocalized electrons, induced in unsaturated molecules by a magnetic field perpendicular to the molecular plane, has been developed to define simple and practical indices of current strength, which provide a reliable measure of aromaticity on the magnetic criterion. Comparison with corresponding ab initio estimates of current strength for a series of aromatic and antiaromatic systems is used to assess the quality of the proposed indices, which are shown to be preferable to others currently used, e.g., out-of-plane components of magnetizability, ξ‖, and NICS‖. It is also shown that these quantities are both biased by spurious geometrical parameters, which limit their practicality. In particular, NICS‖ depends on the inverse of the ring radius and it does not provide acceptable magnetotropicity scales.