Hashemi’s Sanitation Sustainability Index (HSSI) is a community‑based framework for judging the sustainability of sanitation options before they are built. It combines three domains into a single 0–1 score: technical performance (how much water and energy a system uses and how safely it recycles waste), economic viability (relative capital and maintenance costs and whether the system can generate direct benefits), and social considerations (community acceptability and protection of public health). Each component is measured with simple, dimensionless indicators referenced to local norms, then averaged to yield an overall rating that allows straightforward comparison of options. In a case study from suburban Seoul, a conventional septic tank and a resource‑oriented sanitation (ROS) system were evaluated; the ROS system scored higher (HSSI ≈ 0.71 vs. 0.42 for septic) and retained that advantage under 1,000,000‑trial Monte Carlo forecasting (means ≈ 0.67 vs. 0.36), largely due to stronger waste‑recycling potential and possible economic returns despite somewhat lower social scores. The index is designed to be flexible and replicable across settings, though its accuracy depends on locally available technical, economic, and social data and would benefit from standardized surveys for the social measures.