Select land cover or enter mixed land-use areas to look up Rational Method C values and calculate an area-weighted composite runoff coefficient.
For educational purposes only. Not a substitute for professional engineering judgment.
Single Surface Lookup: Look up the runoff coefficient (C value) for a specific surface type or land use category. Includes typical value and range from HEC-22.
Composite (Mixed Use): Calculate the area-weighted average runoff coefficient for drainage areas with multiple land uses or surface types.
Define your subareas and click Calculate Composite C to see results.
| Surface/Land Use Type | C (Min) | C (Typical) | C (Max) |
|---|---|---|---|
| Rooftop (Metal) | 0.95 | 0.95 | 0.95 |
| Rooftop (Gravel) | 0.85 | 0.88 | 0.9 |
| Asphalt/Concrete Pavement | 0.7 | 0.85 | 0.95 |
| Brick | 0.7 | 0.78 | 0.85 |
| Driveways and Walks | 0.75 | 0.8 | 0.85 |
| Lawn (Sandy Soil, Flat 0-2%) | 0.05 | 0.08 | 0.1 |
| Lawn (Sandy Soil, Average 2-7%) | 0.1 | 0.13 | 0.15 |
| Lawn (Sandy Soil, Steep >7%) | 0.15 | 0.18 | 0.2 |
| Lawn (Clay Soil, Flat 0-2%) | 0.13 | 0.15 | 0.17 |
| Lawn (Clay Soil, Average 2-7%) | 0.18 | 0.2 | 0.22 |
| Lawn (Clay Soil, Steep >7%) | 0.25 | 0.3 | 0.35 |
| Business (Downtown) | 0.7 | 0.85 | 0.95 |
Source: HEC-22 (FHWA, 2009), Table 3-1. Showing first 12 entries. Select from dropdown for full list.
| Return Period (years) | 2 | 5 | 10 | 25 | 50 | 100 |
|---|---|---|---|---|---|---|
| Frequency Factor (Cf) | 1 | 1 | 1 | 1.1 | 1.2 | 1.25 |
Source: HEC-22 (FHWA, 2009), Table 3-2. Applied to C values for storms with return periods greater than 10 years.
For drainage areas with multiple land uses, the composite runoff coefficient is calculated as the area-weighted average:
Where:
For design storms with return periods greater than 10 years, apply the frequency adjustment factor: