How Stormwater Credits Work

Stormwater utilities bill properties by the runoff they shed — and offer credits to owners who reduce it with green infrastructure. The arithmetic is simple: runoff is rainfall depth times impervious area, green infrastructure captures part of that volume, and the captured share becomes your credit. This stormwater credit score calculator runs that chain end to end.

The Fee-Savings Angle

Credits convert directly into bill reductions. A 1,000 sq ft rain garden on a 7,000 sq ft impervious lot captures roughly 13% of annual runoff at 90% efficiency — worth about $39 a year on a $300 stormwater bill. Over a decade that is meaningful payback for a single installation.

Programs Vary by Municipality

Credit formulas, caps, and application requirements differ by utility. This calculator uses a transparent default (rainfall-based capture at 90% efficiency) that you can adjust to your local program. Always confirm the final credit percentage with your stormwater authority before budgeting the savings.