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.
Size a rain garden (bioretention area) to capture and infiltrate stormwater runoff. Enter your drainage area, design storm and soil properties to get the required surface area, storage volume and drawdown time, with a check against typical sizing and drawdown criteria.
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)
How rain garden sizing works
Sizing is a storage balance: the rain garden must hold the runoff volume from the design storm in its surface ponding and the void space of its amended soil media. The method follows ASCE MOP 77 and the EPA SWMM bioretention approach in three steps.
1. Composite runoff coefficient — a weighted average of impervious and pervious cover:
C = Ci × fi + Cp × fp
2. Design runoff volume — the depth of rain that becomes runoff over the drainage area:
Vrunoff = C × P × A
3. Required surface area — runoff volume divided by the effective storage depth:
ARG = Vrunoff ÷ (dpond + dsoil × n)
Drawdown time — the hours needed to empty the ponded water — is then checked against the native soil infiltration rate:
tdrain = dpond ÷ f
- C — composite runoff coefficient (dimensionless); Ci, Cp are the impervious and pervious coefficients and fi, fp their area fractions
- P — design rainfall depth (in or mm), e.g. the 1-inch first-flush storm
- A — contributing drainage area (sf or m²)
- dpond — maximum surface ponding depth (typically 6–9 in)
- dsoil — amended soil media depth (typically 18–24 in)
- n — amended soil porosity / void fraction (typically 0.25–0.35)
- f — native soil infiltration rate (in/hr or mm/hr)
The design is judged adequate when the storage-to-runoff ratio is at least 1.0 and the drawdown time meets the target (24–48 hours).
Typical rain garden design parameters
Default and recommended ranges used in this calculator, based on ASCE MOP 77 and common bioretention practice. Confirm against your local stormwater manual.
| Parameter | Symbol | Typical range | Default |
|---|---|---|---|
| Maximum ponding depth | dpond | 6–9 in (150–225 mm) | 6 in |
| Amended soil depth | dsoil | 18–24 in (450–600 mm) | 18 in |
| Amended soil porosity | n | 0.25–0.35 | 0.30 |
| Water quality (first-flush) depth | PWQ | First 1 in (25 mm) | 1.0 in |
| Target drawdown time | tdrain | 24–48 hr | 24 hr |
| Minimum sizing ratio (storage / runoff) | — | ≥ 1.0 (1.2+ preferred) | 1.0 |
Source: ASCE MOP 77 (2006); EPA SWMM Manual (2020). Keep the groundwater table at least 2 ft below the rain garden bottom and avoid sites steeper than about 6%.
Frequently asked questions
How big should a rain garden be?
A common rule of thumb sizes a rain garden at roughly 5–10% of the contributing impervious drainage area, but the area should be confirmed by a storage calculation. This tool computes the required surface area as the design runoff volume divided by the effective storage depth (ponding depth plus amended-soil depth multiplied by its porosity). For a 1,000 sq ft drainage area that is 50% impervious with a 1-inch design storm, 6 in of ponding and 18 in of amended soil at 0.30 porosity, the required area works out to roughly 50 sq ft.
What is the design storm for a rain garden?
Rain gardens are typically sized for the "first flush" water quality storm — usually the first 1 inch (about 25 mm) of rainfall, or the local 1–2 year event. Capturing the first flush treats the most polluted runoff and handles the majority of annual rain events. Larger storms are passed through a designed overflow or bypass; the rain garden is not intended to detain extreme floods.
How long should a rain garden take to drain?
Target a drawdown time of 24 to 48 hours. Draining within 48 hours prevents standing water that breeds mosquitoes and protects most garden plants from prolonged saturation. Drawdown time is estimated as the maximum ponding depth divided by the native soil infiltration rate, so tight clay soils (under about 0.1 in/hr) often exceed the target and may need amended media or an underdrain.
What soil works best for a rain garden?
Native soils that infiltrate at roughly 0.5 in/hr or faster — sand, loamy sand and sandy loam — are well suited to infiltration-based rain gardens. Slower silt and clay soils (clay loam, silty clay, clay) drain too slowly on their own and typically require an engineered bioretention soil mix and a perforated underdrain to meet the drawdown target. Verify the rate with an on-site infiltration test rather than relying on the soil-class default.
Was this calculator helpful?
Last verified: February 2026