Formula & constants
Permeable pavement stores runoff in an open-graded stone reservoir with about 40% void space, then infiltrates it into the subgrade. Contributing run-on area should not exceed the permeable area itself in most designs, reservoir depth typically lands between 6 and 18 inches, subgrade slope must stay under 1%, and the system must draw down within 48 hours to satisfy construction stormwater permits. 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 calculator (Stormwater Runoff Calculator), then paver base calculator (Paver Base 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 deep does the stone reservoir need to be?
Divide the required storage volume by the pavement area and the void ratio, typically 40% for open-graded #57 stone. Most designs land between 6 and 18 inches; the structural requirement is calculated separately and the thicker of the two governs.
How much run-on area can drain onto permeable pavement?
Most manuals limit contributing impervious area to a 1:1 ratio with the permeable surface, and some allow up to 2:1 with pretreatment. Exceeding it overwhelms the reservoir and accelerates surface clogging.
What drawdown time is required?
The water quality volume must infiltrate within 48 hours under construction stormwater general permits, and 24 hours is recommended where discharges reach sensitive waters.
What if my soil infiltrates slowly?
Below about 0.5 inches per hour an underdrain is generally required, and an underdrained system does not earn full volume-reduction credit. Design manuals also cap credited infiltration rates around 1.63 inches per hour.
Can permeable pavement handle truck traffic?
It performs well for light-traffic parking, driveways and overflow areas. Structural performance under high repetitions of heavy trucks has not been consistently demonstrated, so conventional pavement is usually specified for those lanes.
Last verified: — See our methodology.
Planning estimates only — confirm quantities with suppliers and local code before ordering.