Since the Global Crisis, debt sustainability has received increasing attention. This column argues that the maximum sustainable debt level depends negatively on the progressivity of the tax system. The authors estimate that the US is still relatively far from the peak of its Laffer curve and from its maximally sustainable debt level. However, adopting a flat tax would raise the maximum sustainable debt from 330% to more than 350% of benchmark GDP, whereas adopting Danish-style progressivity would lower it to less than 250%.