What This Solves
Sizes a shallow vegetated depression (rain garden or bioretention cell) to capture, temporarily store, and infiltrate stormwater runoff from a contributing drainage area.
Best Used When
- You are designing a residential rain garden to manage roof or driveway runoff
- You need to calculate the required surface area, ponding depth, and soil media depth for a bioretention feature
- You want to verify that the rain garden will drain within a target time (typically 24-48 hours)
Do NOT Use When
- You need a formal bioretention system with underdrains and layered media — Use Bioretention with Underdrain Calculator
- You are designing a vertical infiltration system (dry well or seepage pit) instead of a surface basin — Use Dry Well Calculator
- You are designing a permeable pavement system rather than a planted basin — Use Permeable Paver Calculator
Key Assumptions
- Infiltration occurs primarily through the bottom of the rain garden at the specified soil infiltration rate
- The amended soil media (if used) has uniform porosity and infiltration capacity
- Runoff volume is based on a single design storm with uniform rainfall
- Drawdown time is calculated assuming constant infiltration (does not account for saturated vs. unsaturated conditions)
- No significant groundwater mounding or seasonal high water table interference
Input Quality Notes
Soil infiltration rate is the most critical input. Field infiltration tests (double-ring infiltrometer or percolation test) are strongly recommended. Use conservative (low) infiltration rates for design to account for long-term clogging.
Calculate Rain Garden Size
For educational purposes only. Not a substitute for professional engineering judgment.
Rain Garden Design Overview
Rain gardens (bioretention areas) capture and infiltrate stormwater runoff through ponding and amended soil media. They provide both water quality treatment and quantity control for the "first flush" of runoff.
- Ponding Storage - Surface depression that temporarily holds water
- Soil Storage - Water held in pore spaces of amended soil
- Infiltration - Water percolates to native soil below
- Drawdown Time - Time to drain (24-48 hours recommended)
Soil Infiltration Rates
| Soil Class | Min (in/hr) | Typical (in/hr) | Max (in/hr) |
|---|---|---|---|
| Sand | 2 | 8 | 12 |
| Loamy Sand | 1 | 4 | 8 |
| Sandy Loam | 0.5 | 2 | 4 |
| Loam | 0.25 | 1 | 2 |
| Silt Loam | 0.15 | 0.5 | 1 |
| Sandy Clay Loam | 0.1 | 0.3 | 0.5 |
| Clay Loam | 0.05 | 0.2 | 0.4 |
| Silty Clay Loam | 0.04 | 0.15 | 0.3 |
| Silty Clay | 0.02 | 0.1 | 0.2 |
| Clay | 0.01 | 0.05 | 0.1 |
Source: EPA SWMM Manual (2020), NRCS Soil Survey
Runoff Coefficients
| Ground Cover | Min C | Typical C | Max C |
|---|---|---|---|
| Impervious Surface | 0.90 | 0.95 | 0.98 |
| Rooftop | 0.85 | 0.90 | 0.95 |
| Pavement/Concrete | 0.85 | 0.90 | 0.95 |
| Gravel Surface | 0.25 | 0.50 | 0.70 |
| Bare Soil | 0.20 | 0.40 | 0.60 |
| Lawn (Poor Condition) | 0.25 | 0.35 | 0.45 |
| Lawn (Good Condition) | 0.10 | 0.20 | 0.30 |
| Meadow/Grass | 0.10 | 0.20 | 0.35 |
| Forest/Wooded | 0.05 | 0.15 | 0.25 |
| Mixed Development | 0.20 | 0.40 | 0.60 |
Source: USDA NRCS TR-55 (1986), ASCE MOP 77 (2006)
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Last verified: February 2026