This dataset provides estimates of residential septic tank densities at 12-digit Hydrologic Unit Code (HUC12), National Hydrography Dataset (NHD Plus version 2.1, 1:100k scale) catchment, and 2020 Census Block Group (version 2021) scale for the conterminous United States. Using United States Legacy Census data from the 1990s and the land use classification from the National Land Cover Database (NLCD), predictive thresholds for wastewater treatment types, such as septic or sewer systems, were developed and applied to the 2020 Census and 2019 NLCD data . The dataset contains three qualitative measures, e.g., High, Medium and Low density estimates, for sewer-supported areas and septic tanks by Census Block Groups using the predictive thresholds. Within areas predicted to be supported by septic tanks, a method was developed to estimate septic tank presence using building footprint datasets from Microsoft and the Federal Emergency Management System (FEMA) and parcel data from the Homeland Infrastructure Foundation-Level Data (HIFLD). The HIFLD parcel datasets contain sensitive information and are subject to data use agreements and restrictions, so individual septic tank locations were summarized by HUC12 watersheds, NHD catchments, and block groups. Densities were calculated as the count of septic tanks per square kilometer. A supplemental layer is included, indicating the completeness of the parcel data at the block group scale, to highlight areas where missing data may lead to possible undercounting of septic tanks. Tabular data files can be joined to corresponding spatial features using the unique IDs provided within the source data (for example FeatureID, HUC12, and GeoID).