The growth rate of the vertical instability of elongated plasmas increases with the internal inductance. An upper limit to the value of the internal inductance is found, based on analytic approximations, and validated through the analysis of a large database of Alcator C-Mod plasmas. The extrapolation to ITER suggests that low-current high-q95 plasmas could exceed the original design value of li and thus be more difficult to stabilize. Metrics to characterize the vertical stability of tokamak plasmas are discussed and calculated for typical and ITER-like C-Mod plasmas. These figures contribute to the assessment of the ITER vertical stabilization loop and the establishment of what performance margin is required.