The runoff coefficient (C) represents the fraction of rainfall that becomes runoff. Values range from 0 to 1, with higher values indicating more impervious surfaces and greater runoff. These coefficients are used with the Rational Method for peak flow estimation in small drainage areas (typically < 200 acres).
Rational Method Equation
Peak Flow (Q):
Q = C i A
Where:
- Q = peak runoff rate (cfs)
- C = runoff coefficient (dimensionless)
- i = rainfall intensity (in/hr)
- A = drainage area (acres)
Note: The formula gives Q in cfs when i is in in/hr and A is in acres (conversion factor 1.008 is approximately 1).
Showing 21 of 21 surface types
| Surface Type ▲ | C Min | C Typical | C Max | Category | Source |
|---|---|---|---|---|---|
Asphalt Driveway/Walks Asphalt driveways, sidewalks, and parking lots | 0.75 | 0.85 | 0.95 | Impervious Surfaces | HEC-22, Table 3-1 |
Asphalt Street Asphalt paved street surface | 0.70 | 0.85 | 0.95 | Impervious Surfaces | HEC-22, Table 3-1 |
Brick Pavement Brick or stone block pavement | 0.70 | 0.78 | 0.85 | Impervious Surfaces | HEC-22, Table 3-1 |
Built-up Roof Built-up or membrane roofing | 0.85 | 0.90 | 0.95 | Impervious Surfaces | HEC-22, Table 3-1 |
Cemeteries Cemeteries with maintained grass | 0.10 | 0.18 | 0.25 | Parks and Open Space | HEC-22, Table 3-1 |
Concrete Driveway/Walks Concrete driveways, sidewalks, and parking lots | 0.80 | 0.90 | 0.95 | Impervious Surfaces | HEC-22, Table 3-1 |
Concrete Street Concrete paved street surface | 0.80 | 0.90 | 0.95 | Impervious Surfaces | HEC-22, Table 3-1 |
Gravel Roof Built-up roof with gravel cover | 0.85 | 0.88 | 0.90 | Impervious Surfaces | HEC-22, Table 3-1 |
Gravel Surface Gravel roads, driveways, and parking areas | 0.25 | 0.40 | 0.60 | Impervious Surfaces | ASCE MOP 77, Table 5-1 |
Lawn (Clay Soil, Flat) Lawn with clay soil, flat slope (0-2%) | 0.13 | 0.15 | 0.17 | Lawns | HEC-22, Table 3-1 |
Lawn (Clay Soil, Moderate) Lawn with clay soil, moderate slope (2-7%) | 0.18 | 0.20 | 0.22 | Lawns | HEC-22, Table 3-1 |
Lawn (Clay Soil, Steep) Lawn with clay soil, steep slope (>7%) | 0.25 | 0.30 | 0.35 | Lawns | HEC-22, Table 3-1 |
Lawn (Sandy Soil, Flat) Lawn with sandy soil, flat slope (0-2%) | 0.05 | 0.08 | 0.10 | Lawns | HEC-22, Table 3-1 |
Lawn (Sandy Soil, Moderate) Lawn with sandy soil, moderate slope (2-7%) | 0.10 | 0.13 | 0.15 | Lawns | HEC-22, Table 3-1 |
Lawn (Sandy Soil, Steep) Lawn with sandy soil, steep slope (>7%) | 0.15 | 0.18 | 0.20 | Lawns | HEC-22, Table 3-1 |
Metal Roof Metal roofing surfaces | 0.90 | 0.95 | 0.95 | Impervious Surfaces | HEC-22, Table 3-1 |
Parks (Grass Cover) Public parks with grass cover | 0.10 | 0.18 | 0.25 | Parks and Open Space | HEC-22, Table 3-1 |
Playgrounds Playgrounds with mixed surfaces | 0.20 | 0.28 | 0.35 | Parks and Open Space | HEC-22, Table 3-1 |
Railroad Yard Railroad yard areas with ballast and gravel | 0.20 | 0.30 | 0.40 | Unimproved Areas | HEC-22, Table 3-1 |
Shingle Roof Composition shingle roofing | 0.75 | 0.85 | 0.90 | Impervious Surfaces | HEC-22, Table 3-1 |
Unimproved/Vacant Land Unimproved or vacant land with natural cover | 0.10 | 0.20 | 0.30 | Unimproved Areas | HEC-22, Table 3-1 |
Frequency Adjustment Factors (Cf)
For return periods greater than 10 years, multiply the runoff coefficient by the frequency factor below. The adjusted C value should not exceed 1.0.
| Return Period (years) | 2 | 5 | 10 | 25 | 50 | 100 |
|---|---|---|---|---|---|---|
| Cf | 1.00 | 1.00 | 1.00 | 1.10 | 1.20 | 1.25 |
Source: HEC-22, Table 3-2. Adjusted C = C x Cf (maximum 1.0)
Slope Adjustment
For pervious surfaces, slope affects runoff coefficients. The lawn values in the table are categorized by slope:
Flat (0-2%)
Lowest C values - maximum infiltration opportunity
Moderate (2-7%)
Intermediate C values - typical suburban conditions
Steep (>7%)
Highest C values - reduced infiltration time
Design Guidance
Composite Runoff Coefficient
For drainage areas with multiple surface types, calculate an area-weighted composite C:
Ccomposite = sum(Ci x Ai) / Atotal
Range Selection
- Use typical values for general design
- Use maximum values for conservative design or critical infrastructure
- Consider soil type, slope, and development intensity when selecting within the range
Method Limitations
- Rational Method is best for small watersheds (< 200 acres)
- Assumes uniform rainfall over the drainage area
- Assumes steady-state conditions (rainfall duration >= time of concentration)
- Does not account for detention/storage effects
Soil Type Considerations
For lawn and vegetated areas, the table provides separate values for sandy (well-drained) and clay (poorly-drained) soils. Select based on the predominant soil type or HSG:
- Sandy: HSG A and B soils
- Clay: HSG C and D soils
Frequently Asked Questions
What is the runoff coefficient for asphalt?
The runoff coefficient (C) for asphalt streets is 0.70–0.95, with a typical design value of 0.85 (HEC-22 Table 3-1). Asphalt driveways, walks, and parking lots use the same 0.75–0.95 range. Because asphalt is nearly impervious, use the higher end for older, well-sealed pavement.
What is a typical runoff coefficient for single-family residential?
Single-family residential areas use a runoff coefficient of 0.30–0.50, with 0.40 typical (HEC-22 Table 3-1). Larger suburban lots run lower (0.25–0.40), while attached townhomes run higher (0.60–0.75). Choose within the range based on lot size and impervious cover.
How do I calculate a composite runoff coefficient?
For a drainage area with multiple surfaces, use an area-weighted average: C_composite = Σ(Cᵢ × Aᵢ) / A_total. Multiply each surface’s C by its area, sum the products, and divide by the total area. The runoff coefficient calculator does this automatically.
Do runoff coefficients change with storm return period?
Yes. For return periods over 10 years, multiply C by a frequency factor (Cf): 1.10 at 25-year, 1.20 at 50-year, and 1.25 at 100-year (HEC-22 Table 3-2). The adjusted value C × Cf is capped at 1.0.
Primary Sources
- FHWA HEC-22 (2009). Urban Drainage Design Manual, 3rd Ed. Tables 3-1, 3-2.
- ASCE MOP 77 (2006). Design and Construction of Urban Stormwater Management Systems. Table 5-1.
- McCuen, R.H. (2005). Hydrologic Analysis and Design, 3rd Ed. Prentice Hall.
Related Calculators & Guides
Need to compute a value? Use the runoff coefficient calculator to build an area-weighted composite C, or read the runoff coefficient selection guide for help choosing values.