An R package implementing the stratification index (Zhou 2012). The package provides two functions, srank(), which returns stratum-specific information, including population share and average percentile rank; and strat(), which returns the stratification index and its approximate standard error. When a grouping factor is specified, strat also provides a detailed decomposition of the overall stratification into between-group and within-group components. Available at https://cran.r-project.org/web/packages/strat.