Building on existing efforts to theorize and measure state capacity, this paper proposes a spatial measure of equity promotion: developmental state capacity (DSC). We operationalize DSC as the spatial distribution and density of capability-enhancing infrastructure in a given territorial unit. Specifically, we develop a composite index of four types of local basic infrastructure: primary schools, community clinics, piped potable water, and electricity. Unlike territorial approaches or population-based development indicators, this index allows researchers to better understand and evaluate the state's role in human development by visualizing and examining the patterned spatial distribution and density of DSC. We illustrate the advantages of the measure through a case study of Johannesburg, South Africa. Using a unique data set, we calculate DSC index scores for over 600 administrative units. We then map the spatial distribution of DSC, identifying spatial clusters, and quantitatively and qualitatively explore the influence of demographic and political factors on its distribution. We conclude with directions for future research uses of the DSC index.