How does permeable pavement reduce runoff?

Permeable pavement lets water pass through the surface into a stone reservoir. Water is temporarily stored in the void spaces and can infiltrate into native soil, reducing surface runoff for storms the system can hold and drain.

What inputs matter most?

The most sensitive inputs are pavement area, design storm depth, stone reservoir depth, void ratio, soil infiltration rate, contributing run-on area, and whether the surface remains unclogged.

Why does the calculator show drawdown time?

Drawdown time estimates how long stored water takes to leave through native soil infiltration after the storm. Slow drawdown can reduce capacity for the next storm and may indicate a need for underdrain or professional review.

Is this an engineering design?

No. This is a planning estimator. It does not replace local stormwater criteria, geotechnical testing, pavement structural design, frost review, overflow routing, underdrain design, or professional engineering.

Are my site measurements uploaded?

No. The calculation runs locally in your browser and does not send your inputs anywhere.

Permeable Pavement Runoff Reduction Calculator

Estimate how much design-storm runoff a porous asphalt, pervious concrete, or permeable paver section can capture. Private by design: runs locally in your browser.

Inputs

Units Pavement type preset Aggregate void ratio

Permeable pavement area

Project footprint, in square feet.

Extra impervious run-on area

Roof, drive, or paved area draining onto the pavement, in square feet.

Existing pavement runoff coefficient

Use 0.90 to 0.98 for asphalt, concrete, and roofs.

Design storm depth

Rain depth for the design event, in inches.

Storm duration

Used to estimate infiltration during the storm, in hours.

Annual rainfall

Optional annualized estimate, in inches/year.

Stone reservoir depth

Open-graded aggregate thickness below pavement, in inches.

Native soil infiltration rate

Measured subgrade infiltration rate, in inches/hour.

Surface clogging allowance

Reduces effective capture capacity for sediment and maintenance uncertainty.

Advanced coefficients

Run-on area runoff coefficient Surface depression storage

Small temporary surface storage, in inches.

Annual estimate method

These values are for screening only. Detailed designs should use local rainfall distributions, soil testing, structural pavement design, overflow routing, and any required underdrain criteria.

Planning-level estimator only. Confirm local requirements, utility conflicts, subgrade suitability, overflow paths, maintenance access, frost or groundwater constraints, and structural pavement design before construction.

Results

What This Calculator Estimates

Baseline design-storm runoff: (pavement area x storm depth x existing coefficient) + (run-on area x storm depth x run-on coefficient).
Reservoir pore storage: permeable pavement area x stone reservoir depth x aggregate void ratio.
Infiltration during storm: permeable pavement area x native soil infiltration rate x storm duration.
Captured volume: the lesser of design-storm inflow and effective capacity after clogging allowance.
Drawdown time: stored water after the storm divided by the native-soil infiltration rate across the pavement footprint.

In imperial mode, 1 inch of rain over 1 square foot is about 0.623 US gallons. In metric mode, 1 mm over 1 square meter is 1 litre.

Assumptions, Sources, And Limits

Last reviewed: June 30, 2026

This tool is for early stormwater screening. It does not model time-series rainfall, pavement hydraulics, surface clogging over years, underdrain flow, groundwater mounding, frost heave, traffic loading, water quality treatment, permits, or flood safety.

EPA National Stormwater Calculator: EPA describes porous pavement as excavated areas filled with gravel and paved with porous concrete, asphalt, or modular blocks. It notes that rainfall normally passes into the gravel storage layer where it can infiltrate at natural soil rates.
EPA modeling context: The same EPA page explains that the National Stormwater Calculator uses SWMM and low impact development controls for screening-level runoff analysis of small to medium sites.
Editable defaults: The default 12 inch stone reservoir, 0.40 void ratio, 0.3 inch/hour soil infiltration, and 15% clogging allowance are starting values only. Replace them with local design standards, measured infiltration, and manufacturer or engineer-reviewed details.
Overflow: Permeable pavement still needs a safe overflow route for storms larger than its capture capacity and for periods when sediment or ice reduces surface intake.

FAQ

What void ratio should I use?

Open-graded stone reservoirs are often estimated around 0.30 to 0.40 void space, but actual values depend on aggregate gradation, compaction, and construction quality. Use the project specification when available.

Why does run-on area matter?

A permeable pavement section may receive water from nearby impervious surfaces. Extra run-on can be useful when designed intentionally, but too much can overwhelm the reservoir or clog the surface faster.

What drawdown time is acceptable?

Local stormwater manuals often set drawdown targets so storage is available for the next storm. This calculator flags drawdown over 48 hours as a planning concern, but your local rule may be stricter.

Does this account for an underdrain?

No. The calculation assumes captured water leaves by native-soil infiltration. If a project includes an underdrain, use local design methods or a stormwater model that can represent drain elevation, outlet control, and downstream capacity.

Can permeable pavement replace flood modeling?

No. This is an event-volume screening tool. Flood risk, peak discharge, overland routing, groundwater effects, and public infrastructure connections need project-specific modeling and local review.