Rainwater Runoff Calculator
Calculate the volume of rainwater runoff in cubic feet for a roof or surface area during rainfall
Input Parameters
Runoff Results
Total Surface Area
0 square feet
Runoff Volume
0 cubic feet
Equivalent Water Volume
0 gallons
Calculation Breakdown
About Rainwater Runoff Calculation
How It Works
This calculator helps you estimate the volume of rainwater runoff from a roof or surface area during rainfall. The calculation follows these steps:
- Calculate the total surface area in square feet
- Convert rainfall from inches to feet (divide by 12)
- Calculate the theoretical volume (area × rainfall depth)
- Apply the runoff coefficient to account for water absorption or loss
- Calculate the final runoff volume in cubic feet
- Convert to gallons (1 cubic foot = 7.48 gallons)
The Runoff Formula
Runoff Volume (cubic feet) = Surface Area (sq ft) × Rainfall Depth (ft) × Runoff Coefficient
Runoff Coefficients
The runoff coefficient represents the percentage of rainfall that becomes runoff. It varies based on surface type, slope, and other factors:
| Surface Type | Typical Coefficient | Description |
|---|---|---|
| Roof | 0.90 – 0.95 | Most roof materials are highly impervious |
| Concrete/Asphalt | 0.80 – 0.95 | Paved surfaces with minimal absorption |
| Gravel | 0.50 – 0.70 | Some water penetrates between gravel particles |
| Lawns, Sandy Soil | 0.10 – 0.35 | Higher absorption rate due to sandy composition |
| Lawns, Heavy Soil | 0.15 – 0.40 | Clay or compacted soils have less absorption |
| Wooded Areas | 0.05 – 0.25 | Trees and undergrowth significantly reduce runoff |
Practical Applications
Rainwater Harvesting
Calculate potential rainwater collection from your roof to size rain barrels or cisterns for storage and reuse.
Rain Garden Sizing
Determine the volume of runoff that needs to be managed to properly size a rain garden or bioretention area.
Drainage System Design
Size gutters, downspouts, and drainage pipes to handle the expected volume of runoff from a design storm.
Stormwater Management
Estimate runoff to plan for flood prevention and erosion control measures on your property.
Important Note
This calculator provides estimates only. Actual runoff volumes may vary based on specific site conditions, including slope, soil saturation, rainfall intensity, and other environmental factors. For critical applications, please consult with a professional engineer or stormwater specialist.