What This Solves
Sizes a dry well (seepage pit) for stormwater infiltration by calculating the required storage volume, infiltration capacity, and drawdown time.
Best Used When
- You need a vertical underground infiltration system for roof or driveway runoff
- You want to calculate the required dry well dimensions to infiltrate a design storm within a target time
- You are evaluating whether site soils can support a dry well system
Do NOT Use When
- You need a linear subsurface drain rather than a point infiltration system — Use French Drain Calculator
- You are designing a surface bioretention basin with vegetation — Use Rain Garden Calculator
- You need an underground detention system with a controlled outlet rather than infiltration — Use Underground Detention Calculator
Key Assumptions
- Infiltration occurs through the sidewalls and bottom of the dry well
- Soil infiltration rate is constant and does not decrease with saturation
- No groundwater mounding interferes with infiltration capacity
- The dry well structure (perforated ring, gravel-filled pit) provides the specified void volume
- Setbacks from foundations, property lines, and water sources are met
Input Quality Notes
Field percolation testing is essential — table-based infiltration rates can be off by an order of magnitude. Test at the actual depth of the proposed dry well bottom. Check for seasonal high groundwater.
Calculate Dry Well Design
For educational purposes only. Not a substitute for professional engineering judgment.
Dry Well Design Overview
Dry wells (infiltration pits) provide temporary storage and ground infiltration for stormwater runoff. They are commonly used for roof drainage, parking lots, and residential stormwater management.
- Storage Volume - Total volume available in backfill voids
- Infiltration Area - Bottom and sidewall area for water to infiltrate
- Drawdown Time - Time to drain stored volume (24-72 hours typical)
- Safety Factor - Applied to soil infiltration rate for conservative design
Soil Infiltration Rates
| Soil Type | Min (in/hr) | Typical (in/hr) | Max (in/hr) |
|---|---|---|---|
| Gravel | 4 | 8 | 20 |
| Sand | 2 | 4 | 8 |
| Loamy Sand | 1 | 2.4 | 4 |
| Sandy Loam | 0.5 | 1 | 2 |
| Loam | 0.25 | 0.5 | 1 |
| Silt Loam | 0.15 | 0.3 | 0.6 |
| Sandy Clay Loam | 0.1 | 0.2 | 0.4 |
| Clay Loam | 0.05 | 0.1 | 0.2 |
| Silty Clay Loam | 0.04 | 0.08 | 0.15 |
| Sandy Clay | 0.02 | 0.05 | 0.1 |
| Silty Clay | 0.01 | 0.03 | 0.06 |
| Clay | 0.005 | 0.02 | 0.05 |
Source: ASCE MOP 77 (2006), adapted from NRCS data
Backfill Void Ratios
| Backfill Type | Min | Typical | Max |
|---|---|---|---|
| Gravel (3/4" - 1.5") | 0.30 | 0.35 | 0.40 |
| Crushed Stone (#57) | 0.35 | 0.40 | 0.45 |
| Open-Graded Aggregate | 0.38 | 0.42 | 0.48 |
| Perforated Chambers/Crates | 0.90 | 0.95 | 0.97 |
Source: ASCE MOP 77 (2006), Design and Construction of Urban Stormwater Management Systems
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Last verified: February 2026