What This Solves
Designs a permeable pavement system by calculating the required base course depth to store and infiltrate stormwater runoff from the pavement surface and any contributing drainage area.
Best Used When
- You are designing a parking lot, driveway, or walkway with permeable pavers, porous concrete, or porous asphalt
- You need to size the aggregate base layer to provide adequate storage for a design storm
- You want to verify infiltration capacity and check whether an underdrain or overflow is needed
Do NOT Use When
- You are designing a vegetated rain garden or bioretention basin instead of pavement — Use Rain Garden Calculator
- You need a subsurface infiltration trench rather than a pavement system — Use Infiltration Trench Calculator
Key Assumptions
- Aggregate base has uniform porosity throughout the depth
- Subgrade soil infiltration rate is constant and does not vary with depth or saturation
- The pavement surface is 100% permeable (no surface ponding or runoff)
- Contributing drainage area runoff is directed to the permeable pavement and not bypassed
- No clogging of the surface or aggregate over time (assumes proper maintenance)
Input Quality Notes
Subgrade infiltration rate is the most important input. Field infiltration testing is essential. For conservative design, use the low end of measured rates and consider a long-term reduction factor (e.g., 50% of initial rate) to account for clogging.
Size a permeable pavement system. Enter your pavement area, contributing drainage area and design storm to get the required aggregate base depth, available storage volume, subgrade infiltration capacity and the time to drain — checked against a 24–72 hour target.
Calculate Permeable Pavement Design
For educational purposes only. Not a substitute for professional engineering judgment.
Permeable Pavement Design Overview
Permeable pavement systems allow stormwater to infiltrate through the surface into an aggregate base for temporary storage and infiltration to subgrade. They combine stormwater management with functional paved surfaces.
- Surface Infiltration - Water passes through paver joints or porous material
- Base Storage - Aggregate voids store water temporarily
- Subgrade Infiltration - Water slowly percolates to native soil
- Drain Time - Should drain within 24-72 hours for water quality
Paver Surface Infiltration Rates
| Paver Type | Min (in/hr) | Typical (in/hr) | Max (in/hr) |
|---|---|---|---|
| Interlocking Concrete Pavers | 10 | 100 | 400 |
| Concrete Grid Pavers | 50 | 200 | 600 |
| Plastic Grid Pavers | 100 | 500 | 1000 |
| Porous Concrete | 50 | 300 | 700 |
| Porous Asphalt | 30 | 200 | 500 |
| Gravel-Filled Pavers | 200 | 500 | 1200 |
Source: ASCE MOP 77 (2006), Manufacturer Specifications
Aggregate Porosity Values
| Aggregate Type | Min | Typical | Max |
|---|---|---|---|
| ASTM #57 Stone | 0.35 | 0.40 | 0.45 |
| ASTM #2 Stone | 0.38 | 0.42 | 0.48 |
| ASTM #3 Stone | 0.38 | 0.42 | 0.46 |
| Crushed Stone | 0.30 | 0.35 | 0.40 |
| Open-Graded Base | 0.32 | 0.38 | 0.42 |
Source: ASCE MOP 77 (2006), ASTM Specifications
How permeable pavement sizing works
A permeable pavement manages stormwater in three stages: water infiltrates through the paver joints or porous surface, is stored temporarily in the voids of an open-graded aggregate base, then percolates slowly into the native subgrade. The design has to provide enough storage for the runoff and drain that storage fast enough to be ready for the next storm.
This tool follows the conservative approach used in ASCE MOP 77: it sizes the base to hold the full runoff volume without crediting infiltration that occurs during the storm, then credits subgrade infiltration only when checking how quickly the system empties afterwards.
Runoff volume to manage
Vrunoff = C × P × Atotal
where C is the runoff coefficient (0.95 for impervious surfaces), P the design storm rainfall depth, and Atotal the pavement area plus any contributing impervious drainage area.
Required base depth
dbase = Vrunoff ÷ (n × Apave)
where n is the aggregate void ratio (porosity) and
Apave the permeable pavement area. Available storage is
V = n × Apave × (dbase + dbedding).
Design subgrade infiltration
fsub = fmeasured ÷ SF
The measured (or table) subgrade rate is divided by a factor of safety SF (typically 2, sometimes 3) to reflect long-term performance after partial clogging.
Drain time
tdrain = Vstorage ÷ (fsub × Apave)
The stored volume should empty within 24–72 hours. Longer drain times indicate the subgrade is too tight and an underdrain is needed.
Subgrade soil infiltration rates
Typical native-soil infiltration rates used for the subgrade. Apply your factor of safety
to these (or, better, use a field-measured rate). The design rate
fsub directly controls drain time and whether full infiltration is
feasible.
| Subgrade soil | Min (in/hr) | Typical (in/hr) | Max (in/hr) |
|---|---|---|---|
| Gravel | 2 | 8 | 20 |
| Sand | 1 | 4 | 8 |
| Sandy Loam | 0.5 | 1.5 | 3 |
| Loam | 0.25 | 0.75 | 1.5 |
| Silt Loam | 0.15 | 0.4 | 0.8 |
| Clay Loam | 0.05 | 0.15 | 0.3 |
| Clay | 0.01 | 0.05 | 0.1 |
Source: EPA SWMM Manual, NRCS Soil Survey (1 in/hr ≈ 25.4 mm/hr). Soils slower than about 0.5 in/hr generally require an underdrain to meet the 72-hour drain-time target.
Worked example
A 500 sf permeable driveway with 200 sf of contributing roof runoff, a 1.5 in design storm, ASTM #57 base (n = 0.40):
- Runoff volume = 0.95 × (1.5 ÷ 12) × (500 + 200) = ≈ 83 cf
- Required base depth = 83 ÷ (0.40 × 500) = 0.42 ft = ≈ 5 in, so a proposed 6 in base (plus a 2 in bedding layer) provides adequate storage
- On sandy loam (1.5 in/hr typical) with a safety factor of 2, the design subgrade rate is 0.75 in/hr, draining the stored volume well within 72 hours
Numbers are indicative; the calculator computes the exact values with your inputs and unit system.
Frequently asked questions
How deep does the aggregate base under permeable pavers need to be?
The base depth is set by storage, not just structure. For a conservative infiltration design the required depth is d = V_runoff / (n × A_pavement), where V_runoff is the runoff volume to be stored, n is the aggregate void ratio (about 0.40 for ASTM #57 stone) and A_pavement is the permeable surface area. A common starting point is a 6–12 in (150–300 mm) open-graded base, then increase depth (or reduce the contributing area) until available storage meets or exceeds the required storage. Structural and frost requirements may force a deeper base than storage alone.
How long should a permeable pavement take to drain?
Aim to empty the stored volume within 24–72 hours. Drain time is estimated as t = V_storage / Q_infiltration, where Q_infiltration = f_subgrade × A_pavement. Draining within this window restores storage before the next storm and protects against standing water and freeze problems. Drain times beyond 72 hours indicate the subgrade is too tight and an underdrain or other outlet is needed.
Why apply a safety factor to the subgrade infiltration rate?
Measured or table infiltration rates degrade over time as fines migrate into the subgrade and the surface partially clogs. The calculator divides the measured rate by a factor of safety (typically 2, with 3 used for finer or higher-risk soils) so the design subgrade rate f_sub = f_measured / SF reflects long-term, not as-built, performance.
Can permeable pavers be used over clay soils?
They can, but clay subgrades infiltrate very slowly (roughly 0.05 in/hr typical, even less after a safety factor), so full infiltration alone usually cannot meet the 72-hour drain target. On tight soils the system is normally designed as a detention/filtration facility with an underdrain that discharges the stored water, rather than relying on infiltration into the subgrade.
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Last verified: February 2026