Formula & constants
Hundreds of US municipalities bill commercial property by impervious area, and most offer credits for on-site retention. Detroit, for example, offers up to an 80% drainage-charge credit — 40% for volume and 40% for peak flow control. Savings equal the reduced impervious billing plus the credit percentage applied to the remainder. All default values are editable — when your supplier's data sheet or local code says otherwise, use that number.
Where this fits in your project
This tool is one step in a bigger job. Once you have these numbers, stormwater runoff estimate tool (Stormwater Runoff Calculator), then permeable pavement reservoir estimate tool (Permeable Pavement Reservoir Calculator). For the full sequence see the project workflows, compare approaches on the comparison pages, and check local factors in the state guides.
Frequently asked questions
What is a stormwater utility fee?
A municipal charge based on how much impervious surface a property has, since impervious area drives the runoff the storm sewer system must handle. It is often billed monthly per 1,000 square feet of hard surface.
How large can stormwater credits be?
They vary widely by city. Detroit's programme offers up to 80% off the drainage charge — up to 40% for volume removed from the system and up to 40% for controlling peak flow rate, evaluated case by case.
How long does permeable pavement take to pay back?
It depends on your local fee rate and credit percentage. Divide the permeable installation premium by annual fee savings; in cities with high fees and generous credits the payback is often under a decade, and the pavement lasts longer than that.
Do I have to apply for the credit?
Almost always. Credits typically require a documented design, proof of storage capacity, and an ongoing maintenance commitment including regular vacuuming to prevent clogging.
What else reduces the fee besides permeable pavement?
Disconnecting downspouts to pervious areas, rain gardens, cisterns and green roofs all reduce billable impervious area or earn retention credits under most programmes.
Last verified: — See our methodology.
Planning estimates only — confirm quantities with suppliers and local code before ordering.