Stormwater Runoff Volume Estimator (Simplified)
Estimate stormwater runoff volume for a design rainfall event using a simple depth × area × runoff coefficient model. For educational and early planning use only, not a substitute for detailed drainage or flood design.
Drainage Area
Design Storm Rainfall
Runoff Coefficient
Typical values: 0.1–0.3 for permeable/vegetated areas, 0.5–0.7 for mixed surfaces, 0.7–0.95 for highly impervious surfaces (roofs, pavements).
Estimate Stormwater Runoff Volume for Your Site
Enter your drainage area, rainfall depth, and a runoff coefficient to see an approximate runoff volume for your design event.
Tip: Common runoff coefficients range from 0.1–0.3 for vegetated areas to 0.7–0.95 for impervious surfaces like parking lots and rooftops.
Understanding Stormwater Runoff
What is Stormwater Runoff Volume?
Stormwater runoff volume is the amount of water that flows over land surfaces during and after a rainfall event. When rain falls on a drainage area, some water infiltrates into the soil, some evaporates, and the remainder becomes surface runoff that must be managed through drainage systems, detention basins, or natural channels.
How This Simplified Estimator Works
This tool uses a basic Depth × Area × Runoff Coefficient model:
The result is an approximate volume of water that becomes surface runoff. If you specify a retention fraction (for onsite storage or infiltration), the effective runoff coefficient is reduced accordingly.
What is a Runoff Coefficient?
The runoff coefficient (C) represents the fraction of rainfall that becomes surface runoff rather than infiltrating into the ground or being retained. It typically ranges from 0 to 1:
- 0.10–0.30: Permeable surfaces like lawns, forests, and vegetated areas
- 0.30–0.50: Mixed surfaces with some pavement and landscaping
- 0.50–0.70: Suburban areas with moderate imperviousness
- 0.70–0.95: Highly impervious surfaces like parking lots, rooftops, and concrete
Common Ranges for Different Surface Types
| Surface Type | Typical C Range |
|---|---|
| Forest / Natural Woodland | 0.05–0.25 |
| Lawns / Parks (well-drained) | 0.10–0.35 |
| Agricultural Land | 0.20–0.50 |
| Single-Family Residential | 0.30–0.50 |
| Multi-Family Residential | 0.50–0.70 |
| Commercial / Industrial | 0.70–0.90 |
| Asphalt / Concrete Pavement | 0.85–0.95 |
| Rooftops | 0.75–0.95 |
Limitations of Simplified Models
This tool provides a rough approximation only. Real stormwater modeling must account for many factors not considered here:
- Soil infiltration rates and antecedent moisture conditions
- Rainfall intensity and duration (hyetograph shape)
- Time of concentration and travel time through the watershed
- Slopes, drainage networks, and flow routing
- Evapotranspiration and interception losses
- Storage in depressions, channels, and detention facilities
How to Use Results for High-Level Thinking Only
Use these estimates for:
- Early-stage feasibility discussions about site development
- Comparing relative runoff impacts of different land uses
- Educational understanding of how runoff scales with area and rainfall
- Initial conversations with engineers about stormwater management needs
Do NOT use these results for: final engineering design, detention basin sizing, flood analysis, permit applications, or any regulatory compliance documentation.
Important: This calculator is for educational and early planning purposes only. It does not replace detailed hydrologic modeling, civil engineering design, or professional consultation. Always work with licensed engineers and local stormwater authorities for real projects.
Frequently Asked Questions
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