Permeable Pavement Runoff & Storage Calculator

Evaluate an existing section or size the pavement area or stone reservoir depth for a target storm. Estimates runoff captured, overflow, required stone depth, loading ratio, slope-adjusted storage, and drawdown for porous asphalt, pervious concrete, and permeable pavers. Runs locally in your browser.

Use this calculator to:

check whether a proposed permeable pavement section captures a target event;
find a planning-level pavement area or aggregate depth for a capture target;
compare full-infiltration, underdrained, and no-infiltration detention scenarios.

Choose a workflow

1. Project area

Units (required)

Permeable pavement area (required)

Footprint in ft².

Extra impervious run-on area (optional)

Roof or paving draining onto the surface, in ft².

Local run-on loading-ratio limit (required)

Extra impervious area ÷ permeable area. Default 5:1 is a screening value from the Minnesota Stormwater Manual; local rules control.

Existing-surface runoff coefficient (required)

Use a locally approved coefficient. Smooth roofs and paving are often screened near 0.90–0.98.

Run-on runoff coefficient (required)

Fraction of rain on the contributing area that reaches the pavement.

2. Design storm

Rainfall input source (required)

Open NOAA PFDS, look up the project point, then enter the published depth and provenance below.

Location or station (required)

Source retrieval date (required)

Average recurrence interval (required)

Design storm depth (required)

Enter the point rainfall depth in inches.

Storm duration, hours (required)

Duration must match the selected source value; it sets the during-storm drainage window.

3. Pavement section

Pavement surface type (required)

Surface choice sets only a starting surface acceptance rate and surface-course storage—not aggregate porosity.

Surface-course storage (required)

Drainable pore/depression storage in the wearing course, in inches. Estimate thickness × connected drainable void fraction; EPA guidance describes 4–6 in surfaces with 10–25% void space. Confirm the product detail.

Clean surface acceptance rate (required)

In inches/hour. Use a maintained field-test or specification value; hydraulic acceptance is distinct from soil infiltration.

Surface acceptance derating, % (required)

Conservative reduction for aging/clogging. It affects surface intake only—not stone porosity or soil rate.

Aggregate reservoir porosity (required)

Neutral custom default 0.40. Open-graded stone is commonly screened around 0.30–0.42 in manual examples; use tested/project-specified compacted porosity.

Aggregate reservoir depth (required)

Open-graded stone thickness in inches; 6–24 in is a broad planning range, not a design rule. See the cited EPA section guidance.

Advanced storage, slope, and initial-condition assumptions

Initial reservoir water, % full (required)

Use a wet antecedent value when the prior storm has not drained.

Reservoir-bottom slope, % (required)

Use the subsurface reservoir slope, not merely the finished surface slope.

Downslope flow-path length (required)

In feet. Used to estimate storage lost to elevation change.

Check dams or terraces create level storage cells (optional)

4. Soil and drainage

Design scenario (required)

Underdrain calculations are simplified planning estimates, not outlet-control design.

Conservative subgrade design infiltration (required)

In inches/hour. Field-test at reservoir invert, then apply the local safety factor. Example default is half a 0.30 in/hr measured rate, consistent with DC guidance.

Local maximum drawdown, hours (required)

Choose the applicable local recovery criterion.

Groundwater/bedrock separation (optional)

In feet. This is reported but not given a universal pass/fail threshold; verify local minimums and seasonal high groundwater.

Optional rough annual rainfall scenario

Show an equal-event annual scenario (optional)

Warning: annual rainfall depth alone cannot reliably predict annual runoff reduction. This optional scenario assumes annual rainfall arrives as identical, fully recovered design events. Use continuous rainfall and time-step recovery modeling for annual performance claims.

Planning-level estimator. Confirm local stormwater criteria, geotechnical testing, structural and frost design, overflow routing, utilities, groundwater, maintenance access, and permits before construction.

Decision summary

Event water-balance method

The calculator uses a transparent, planning-level static event balance. Each release term is capped by water available, so storage and infiltration are not counted twice.

Event inflow: rain × pavement area + rain × run-on area × run-on coefficient.
Surface accepted: lesser of event inflow and pavement area × clean acceptance rate × (1 − surface derating) × storm duration. Excess is immediate surface bypass.
Adjusted storage: area × [surface-course storage + stone depth × porosity × slope factor] − initial stored water.
During-storm release: soil infiltration plus any simplified underdrain release, capped by accepted inflow.
Overflow: surface bypass plus accepted water remaining after during-storm release and available storage.
Post-storm drawdown: stored volume divided between soil-only sump storage and storage reachable by soil plus underdrain.

In imperial mode, 1 inch over 1 square foot is 0.623 US gallons. In metric mode, 1 mm over 1 square metre is 1 litre.

Worked driveway example

The default example uses 600 ft² of porous asphalt, no run-on, a 2 in / 24 hr event, 0.25 in surface-course storage, 12 in of stone at 0.40 porosity, a conservative soil rate of 0.15 in/hr, a level dry reservoir, and a 50% derated 100 in/hr surface rate.

1. Rainfall volume
600 × 2 × 0.6234 = 748.1 gal. The conventional 0.95 runoff baseline is 710.6 gal.

2. Storage
Effective storage depth is 0.25 + 12 × 0.40 = 5.05 in, or about 1,888 gal.

3. During-storm infiltration
600 × 0.15 × 24 × 0.6234 = 1,346.5 gal, capped at the 748.1 gal actually accepted.

4. Overflow and reduction
Surface acceptance is far above inflow; infiltration handles the accepted event. Overflow is 0 gal and reduction versus the 710.6 gal baseline is 100%.

5. Drawdown
No water remains stored at event end, so modeled post-storm drawdown is 0 hr. This does not remove the need for field verification.

What changes?
Adding run-on raises the loading ratio and inflow. Reducing the design soil rate below 0.15 in/hr shifts more water into storage, extends drawdown, and may produce overflow once storage fills.

Method, sources, and limitations

Method v2.0 · updated 14 July 2026

Review disclosure: Prepared and editorially reviewed by the Starlight Tools team. No licensed professional engineer review is claimed. Results are engineering-adjacent planning estimates, not construction documents or compliance determinations.

Rainfall source: NOAA Precipitation Frequency Data Server provides NOAA Atlas 14 point precipitation-frequency estimates and confidence intervals. Enter the location-specific value; the calculator does not automatically retrieve it.
Loading and sizing: the Minnesota Stormwater Manual example identifies area, reservoir depth, media porosity, soil infiltration, and drawdown as key inputs and uses a 5:1 at-grade contributing impervious-area screening limit.
Design-rate and underdrain context: the DC permeable pavement guidance describes conservative design infiltration at one-half a measured rate and underdrain/sump configurations. That safety factor is an example, not a universal requirement.
Slope and drawdown: DDOT green infrastructure standards describe level/terraced profiles or check dams and project-specific drawdown criteria.
Cold climate and site constraints: the EPA Green Streets Handbook discusses groundwater, steep slope, low-permeability soil, traffic, and cold-climate design responses.

Limitations: This is a static event-volume model. It does not route a hydrograph; simulate rainfall time series, groundwater mounding, exfiltration geometry, pipe/orifice hydraulics, or clogging over time; prove structural capacity, water-quality treatment, peak-flow compliance, or flood safety; or replace surveyed elevations and local review. Slope storage is a conservative geometric screening factor and underdrain flow is a user-entered equivalent rate.

Frequently asked questions

What is the difference between permeable, pervious, and porous pavement?

Permeable pavement is the umbrella term. Pervious concrete, porous asphalt, and permeable interlocking pavers are different surface systems that pass water to an aggregate reservoir.

What is a typical permeable pavement sub-base depth?

A 6 to 24 inch open-graded reservoir is a useful screening range, but hydraulic storage, structural loading, frost depth, utilities, and local details control the final thickness.

What soil infiltration rate is acceptable?

There is no universal minimum. Use field testing at the reservoir invert and the conservative design rate required locally; some manuals use one-half the measured rate. Slow soils commonly require an underdrain.

What is the maximum run-on loading ratio?

The limit is jurisdiction-dependent. This calculator defaults to a 5:1 extra impervious run-on area to permeable area screening limit based on Minnesota guidance and lets you change it.

Should permeable pavement draw down in 24 or 48 hours?

Local criteria differ. This tool reports both 24- and 48-hour status; the District of Columbia guidance cited here uses a 24-hour minimum and 48-hour maximum drawdown window for its specified design volume.

Can permeable pavement be installed on a slope?

Yes, within local limits, but a sloped reservoir can lose usable storage. Level bottoms, check dams, baffles, or terraces are commonly used; this calculator derates storage when the bottom is not controlled.

How much separation is needed above groundwater or bedrock?

Required vertical separation is jurisdiction-dependent and must be measured from the reservoir bottom. Do not use a universal value; check the local manual and consider lining or underdraining where infiltration is unsuitable.

Does permeable pavement work in freeze-thaw climates?

It can when the section drains, materials and structure are designed for the climate, and winter sand is avoided. Frost, snow storage, deicing, and plowing details still need local review.

How often does permeable pavement need maintenance?

Inspect at least annually and after early major storms, keep sediment and winter sand off the surface, and vacuum sweep at the frequency required by the manufacturer or local plan. Restore the surface when ponding indicates reduced acceptance.

Does permeable pavement reduce peak flow as well as runoff volume?

Storage and controlled release can reduce and delay peak flow, but this event-volume calculator does not route a hydrograph. Use a time-step model for peak-discharge or flood claims.