What This Solves
Looks up standard runoff coefficients (C values) for the Rational Method based on land use and surface type.
Best Used When
- You need a runoff coefficient for the Rational Method and have clear land use categories
- You are working on a small urban or suburban site with well-defined surface types
- You want to calculate a composite C value for a drainage area with multiple surface types
Do NOT Use When
- You are working on a larger watershed (over 200 acres) where SCS methods are more appropriate — Use SCS Curve Number Calculator
- You need to estimate peak flow directly without manual C value lookup — Use Rational Method Calculator
Key Assumptions
- The provided C values are typical ranges from published literature
- C values account for both direct runoff and infiltration losses
- C values remain constant during the storm event
- For composite areas, the area-weighted average C is appropriate
- Frequency adjustment factors can be applied to C for larger return periods
Input Quality Notes
Published C values are general guidelines. Adjust for local conditions such as soil permeability, antecedent moisture, and surface condition. Field verification is recommended for critical projects.
Try a Common Scenario
Click to pre-fill the calculator with realistic values.
For educational purposes only. Not a substitute for professional engineering judgment.
Calculation Mode
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.
Ready to Calculate
Select a surface type and click Look Up C Value to see results.
Runoff Coefficient Reference Values
| 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.
Frequency Adjustment Factors
| 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.
Composite Runoff Coefficient Formula
For drainage areas with multiple land uses, the composite runoff coefficient is calculated as the area-weighted average:
Where:
- Ci = Runoff coefficient for subarea i
- Ai = Area of subarea i
- Atotal = Total drainage area
For design storms with return periods greater than 10 years, apply the frequency adjustment factor:
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Last verified: February 2026