| Scenario | Drainage area (m²) | Rain (mm) | C | Reservoir depth (mm) | Porosity | Infiltration (mm/hr) | Drain time (hr) | Safety | Required area (m²) |
|---|---|---|---|---|---|---|---|---|---|
| Parking bay | 900 | 20 | 0.90 | 250 | 0.40 | 10 | 24 | 1.20 | ~67 |
| Drive aisle | 1400 | 25 | 0.85 | 300 | 0.35 | 8 | 24 | 1.25 | ~124 |
| Walkway | 300 | 15 | 0.80 | 200 | 0.40 | 15 | 12 | 1.10 | ~18 |
1) Runoff volume
V = Ad × (P / 1000) × C × (Capture% / 100)
- V = runoff volume to manage (m³)
- Ad = drainage area (m²)
- P = rainfall depth (mm)
- C = runoff coefficient (0–1)
2) Capacity per unit pavement area
Cap = (D × n / 1000) + (i × t / 1000)
- D = reservoir depth (mm), n = porosity
- i = infiltration rate (mm/hr), t = drain time (hr)
3) Required permeable pavement area
Areq = (V / Cap) × SF
- SF = safety factor for uncertainty and performance decline
- Select your units and enter the drainage area contributing runoff.
- Enter the design rainfall depth and a realistic runoff coefficient.
- Set reservoir depth and porosity based on your section details.
- Provide infiltration rate and a drain time target used for compliance.
- Optionally enter pavement geometry to compare provided vs required area.
- Press Calculate, then export CSV or PDF for records.
Design intent and capture target
Permeable pavement area is typically sized to capture a defined portion of runoff from a contributing drainage area. Common capture targets range from 70% to 100% of the design event volume, depending on local stormwater rules and overflow allowances. A higher capture percentage increases required area but reduces bypass flow and improves pollutant removal through filtration and storage.
Rainfall depth and runoff coefficient selection
Use a rainfall depth tied to your compliance event (for example, 15–30 mm for water-quality sizing in many manuals). Runoff coefficients should reflect upstream surfaces: roofs and new concrete often fall around 0.85–0.95, dense asphalt 0.80–0.90, and mixed landscaped catchments 0.20–0.50. Conservative inputs help account for clogging and partial connectivity.
Reservoir storage and porosity
The reservoir course stores water within voids. Typical base thickness used for storage is 150–450 mm, while effective porosity commonly ranges from 0.30 to 0.45 for open-graded aggregates. Storage per square meter equals reservoir depth × porosity, so increasing thickness or selecting higher-void stone provides more capacity without expanding footprint.
Infiltration rate and drain-time checks
Infiltration rate should come from field testing and be reduced for design (for example, using a safety factor or conservative percentile). Many specifications require drain-down within 24–48 hours to avoid prolonged saturation and maintain structural performance. In this calculator, infiltration contributes an additional depth over the chosen drain time, complementing reservoir storage.
Example data set and computed result
| Drainage area | 800 m² | Rainfall depth | 25 mm |
|---|---|---|---|
| Runoff coefficient | 0.85 | Capture | 100% |
| Reservoir depth | 250 mm | Porosity | 0.35 |
| Infiltration rate | 12 mm/hr | Drain time | 24 hr |
| Safety factor | 1.20 | Required area (approx.) | 205 m² |
With these inputs, runoff volume is about 17.0 m³. Storage plus infiltration provides about 0.0995 m³ per m², producing roughly 171 m² before safety factor and about 205 m² after applying it.
What does this calculator determine?
It estimates the permeable pavement surface area needed to capture a selected portion of runoff from a contributing drainage area, using rainfall depth, runoff coefficient, reservoir storage, infiltration, and a safety factor.
How should I pick a runoff coefficient?
Match it to the dominant upstream surface. Impervious roofs and pavements are often 0.80–0.95, while landscaped areas may be 0.20–0.50. If the catchment is mixed, use a weighted average based on area.
Which infiltration rate should I enter?
Use a conservative design value from field tests, not the best-case result. Many projects apply reductions for variability, clogging potential, and seasonal effects. When in doubt, use the lower bound accepted by your reviewer.
What drain time should I use?
Enter the maximum drain-down time required by your standard or permit, commonly 24 or 48 hours. A longer drain time increases the infiltration contribution, but you should still verify structural and groundwater constraints separately.
Does porosity represent the surface or the base?
The porosity input represents the effective void ratio of the reservoir layer that stores water. It is not the same as surface joint voids. Use values supported by your aggregate gradation and compaction method.
How do I account for clogging and maintenance?
Use a safety factor, conservative infiltration, and realistic capture targets. Plan routine vacuum sweeping and sediment control upstream. If your site has high fines, consider pretreatment or a higher safety factor to protect long-term performance.
What’s included in the exports?
The CSV and PDF exports include your inputs, computed runoff volume, storage and infiltration capacities, required area, and any provided geometry checks. Save the PDF for submittals and use the CSV for spreadsheets or design logs.