Formula & constants
Every 1,000 square feet of impervious surface sheds about 623 gallons per inch of rainfall. Runoff equals area × rainfall in inches × 0.623 gallons, adjusted by a runoff coefficient near 0.95 for pavement and roofs and about 0.15 for lawn. Most water-quality rules size storage for the first inch of rain, which carries the majority of pollutants. 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, french drain calculator (French Drain Calculator), then permeable pavement reservoir calculator (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
How do I calculate stormwater runoff from my property?
Measure all impervious surfaces — roof, driveway, walks, parking — in square feet, multiply by rainfall depth in inches, then by 0.623 to get gallons. Pervious areas contribute far less, roughly 15% of the same figure.
What impervious percentage triggers stormwater requirements?
It varies by jurisdiction, but above 50% impervious cover many programmes impose management requirements, and LEED credit 6.1 becomes available for reducing runoff rate and quantity by at least 25%.
How much storage do I need for the design storm?
Divide the design-storm runoff in gallons by 7.48 to get cubic feet of required storage. Most water-quality standards use the first inch of rainfall as the design event.
Does permeable pavement reduce my runoff?
Substantially. Permeable systems have runoff coefficients similar to grass, roughly 0.15 to 0.6, and for the most frequent storms well-designed installations have reduced runoff to zero.
Why does the first inch of rain matter most?
It washes the accumulated pollutants off surfaces. Capturing and treating that first flush delivers most of the water-quality benefit for a fraction of the storage a full flood-control design would need.
Last verified: — See our methodology.
Planning estimates only — confirm quantities with suppliers and local code before ordering.