Look up runoff coefficients (C values) for the Rational Method or calculate composite coefficients for mixed land use drainage areas. Free online tool for drainage engineers.
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.
Select a surface type and click Look Up C Value 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:
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