When the Human Development Index (HDI) was introduced in the 1990s, it was an important step toward a more sensible measure of progress, one defined less by GDP growth and more by social goals. But the limitations of HDI have become clear in the 21st century, given a growing crisis of climate change and ecological breakdown. HDI pays no attention to ecology, and retains an emphasis on high levels of income that – given strong correlations between income and ecological impact – violates sustainability principles. The countries that score highest on the HDI also contribute most, in per capita terms, to climate change and other forms of ecological breakdown. In this sense, HDI promotes a model of development that is empirically incompatible with ecological stability, and impossible to universalize. In this paper I propose an alternative index that corrects for these problems: the Sustainable Development Index (SDI). The SDI retains the base formula of the HDI but places a sufficiency threshold on per capita income, and divides by two key indicators of ecological impact: CO2 emissions and material footprint, both calculated in per capita consumption-based terms and rendered vis-à-vis planetary boundaries. The SDI is an indicator of strong sustainability that measures nations’ ecological efficiency in delivering human development.