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.
Look up the runoff coefficient (C value) for a single surface type, or calculate the area-weighted composite coefficient for a mixed land use drainage area. The C value is the input you need for the Rational Method (Q = C·i·A) to estimate peak stormwater flow.
Almost no real drainage area is a single surface. A typical lot or catchment mixes roof, pavement, and lawn, and each drains differently. The composite runoff coefficient blends those surfaces into one number by weighting each surface’s C value by its share of the total area — so a mostly-paved site produces a higher C (more runoff) than a mostly-lawn site of the same size. Enter your surfaces below to get the weighted C, or read the selection guide to learn how to defend your value, and the reference tables for every published C value.
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:
How the runoff coefficient works
The runoff coefficient C is the fraction of rainfall that becomes surface runoff. It is the key land-use parameter in the Rational Method, the most widely used formula for estimating the peak discharge from small drainage areas:
Where:
- Q = peak runoff rate (cubic feet per second, cfs, for imperial units; m³/s for metric)
- C = runoff coefficient (dimensionless, 0 to 1)
- i = rainfall intensity for the design storm (in/hr or mm/hr)
- A = contributing drainage area (acres or hectares)
For an area with more than one surface type, this tool computes the area-weighted composite coefficient:
For design storms rarer than the 10-year event, a frequency adjustment factor Cf is applied to account for the reduced infiltration capacity during intense storms, with the result capped at 1.0:
Coefficient values, the composite weighting equation, and the frequency factors are taken from FHWA HEC-22 (Urban Drainage Design Manual, 2009), Tables 3-1 and 3-2.
Runoff coefficients by surface type
Typical C values and their published ranges for common surfaces and land uses. Select toward the high end of the range for steeper slopes, denser development, or tighter (clay) soils.
| Surface / land use | C (min) | C (typical) | C (max) |
|---|---|---|---|
| Rooftop (metal) | 0.95 | 0.95 | 0.95 |
| Asphalt / concrete pavement | 0.70 | 0.85 | 0.95 |
| Driveways and walks | 0.75 | 0.80 | 0.85 |
| Business (downtown) | 0.70 | 0.85 | 0.95 |
| Industrial (heavy) | 0.60 | 0.75 | 0.90 |
| Apartment dwelling areas | 0.50 | 0.60 | 0.70 |
| Residential (multi-units, detached) | 0.40 | 0.50 | 0.60 |
| Residential (single-family) | 0.30 | 0.40 | 0.50 |
| Residential (suburban) | 0.25 | 0.33 | 0.40 |
| Playgrounds | 0.20 | 0.28 | 0.35 |
| Lawn (clay soil, average 2–7%) | 0.18 | 0.20 | 0.22 |
| Parks, cemeteries | 0.10 | 0.18 | 0.25 |
| Lawn (sandy soil, flat 0–2%) | 0.05 | 0.08 | 0.10 |
| Woodland (flat) | 0.05 | 0.10 | 0.15 |
Source: FHWA HEC-22 (2009), Table 3-1. The interactive calculator above includes the full 25-surface table, including additional lawn slope and soil categories. Select within each range based on site-specific slope, soil, and imperviousness.
Frequency adjustment factors (Cf)
Multiply the base coefficient by Cf for storms rarer than the 10-year event. The adjusted coefficient C × Cf is never allowed to exceed 1.0.
| Return period (years) | 2 | 5 | 10 | 25 | 50 | 100 |
|---|---|---|---|---|---|---|
| Frequency factor Cf | 1.0 | 1.0 | 1.0 | 1.1 | 1.2 | 1.25 |
Source: FHWA HEC-22 (2009), Table 3-2.
Worked example: composite C
A 5-acre site drains to a single outlet and contains two surface types:
- 2.0 acres of asphalt parking (C = 0.85): weighted contribution = 2.0 × 0.85 = 1.70
- 3.0 acres of single-family residential (C = 0.40): weighted contribution = 3.0 × 0.40 = 1.20
- Composite C = (1.70 + 1.20) / 5.0 = 2.90 / 5.0 = 0.58
- For a 25-year design storm, Cadj = 0.58 × 1.1 = 0.64
This composite C of 0.58 (or 0.64 for the 25-year storm) is then used directly in the Rational Method to find the peak flow Q.
Frequently asked questions
What is a runoff coefficient (C value)?
The runoff coefficient C is a dimensionless number between 0 and 1 that represents the fraction of rainfall on a surface that becomes surface runoff rather than infiltrating, evaporating, or being stored. A value of 0 means all rainfall is absorbed, while 1.0 means all rainfall runs off. It is the C term in the Rational Method, Q = C·i·A. Impervious surfaces such as asphalt and metal roofs have high values (0.85–0.95), while pervious surfaces such as flat sandy lawns and woodland are low (0.05–0.15).
How do you calculate a composite (weighted) runoff coefficient?
For a drainage area with several surface types, the composite coefficient is the area-weighted average: C_composite = (C₁A₁ + C₂A₂ + … + CₙAₙ) / A_total. You multiply each subarea's C value by its area, sum those products, and divide by the total area. For example, 2 acres of asphalt (C = 0.85) plus 3 acres of single-family residential (C = 0.40) gives (2×0.85 + 3×0.40) / 5 = 2.90 / 5 = 0.58.
When do I apply a frequency adjustment factor (Cf)?
The frequency adjustment factor Cf accounts for the reduced infiltration capacity during rarer, more intense storms. Per HEC-22 Table 3-2 it is 1.0 for return periods of 2, 5, and 10 years (no adjustment), 1.1 for the 25-year storm, 1.2 for the 50-year storm, and 1.25 for the 100-year storm. The adjusted coefficient is C_adj = C × Cf, capped at a maximum of 1.0.
What is a typical runoff coefficient for residential and paved areas?
From HEC-22 Table 3-1, single-family residential areas typically use C ≈ 0.40 (range 0.30–0.50), suburban residential ≈ 0.33, and apartment areas ≈ 0.60. Asphalt and concrete pavement is about 0.85 (range 0.70–0.95), metal roofs about 0.95, and driveways and walks about 0.80. Always select within the published range based on slope, soil, and degree of development.
Related tools
Reference and further reading
Need the full lookup tables or a deeper explanation of how to pick the right C value for your site? These pages go beyond the calculator.
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Last verified: February 2026