Composite Runoff Coefficient Calculator
Area-weighted Rational Method runoff coefficient across up to six surface types. Enter each surface's C and area; the calculator returns the weighted C, total area, and percent impervious, ready to carry into Q = CiA. Look up C values on the runoff coefficient reference card.
Leave unused surfaces blank. The impervious fraction counts surfaces entered with C ≥ 0.85 (pavement/roof convention). Frequency factors follow FHWA HEC-22 / ASCE practice: published C values represent ~2–10-yr storms; rarer storms saturate pervious cover and run off harder. Check your local drainage manual — many jurisdictions mandate specific C tables and factors.
Common runoff coefficients
| Surface | C |
|---|---|
| Asphalt / concrete pavement | 0.85–0.95 |
| Roofs | 0.85–0.95 |
| Gravel roads / parking | 0.35–0.70 |
| Lawn, sandy soil, flat (<2%) | 0.05–0.10 |
| Lawn, sandy soil, steep (>7%) | 0.15–0.20 |
| Lawn, heavy (clay) soil, flat (<2%) | 0.13–0.17 |
| Lawn, heavy (clay) soil, steep (>7%) | 0.25–0.35 |
| Woods / undeveloped | 0.05–0.25 |
| Cultivated fields | 0.30–0.60 |
Full tables including composite land-use values are on the runoff coefficient cheat sheet.
Worked example
Example — 12-acre commercial site
References: ASCE/WEF (1992). Design and Construction of Urban Stormwater Management Systems. FHWA (2009). Urban Drainage Design Manual, HEC-22, Chapter 3. Chow, Maidment & Mays, Applied Hydrology, Table 15.1.1.
Common mistakes
- Using the same C for every storm. Tabulated C values are typically for the 2- to 10-year storm. Many manuals apply a frequency factor for less frequent storms (often 1.1 for 25-year, 1.2 for 50-year and 1.25 for 100-year), with C × factor capped at 1.0.
- Area-weighting C when one surface drains through another. If roof runoff spills onto a lawn, that water may infiltrate. Composite C assumes every area drains directly to the design point.
- Ignoring the time of concentration of the impervious part. A small, directly connected paved area can produce a higher peak at a short tc than the whole site at its longer tc. Check both.
- Choosing lawn C without checking the soil. Clay-soil lawn has two to three times the C of sandy-soil lawn at the same slope.
When this calculator doesn’t apply
Composite C is for the Rational Method, which is generally limited to small drainage areas, often under 100–200 acres depending on the agency. Larger watersheds, storage design and volume-based requirements should use NRCS curve-number methods or hydrograph routing. Use the C values and limits in the governing local drainage manual where one exists.
Related tools
- Runoff coefficient reference card — full C tables by surface and land use
- Rational method — peak flow Q = CiA with the weighted C
- Composite curve number — the NRCS equivalent for runoff volume
- Time of concentration — sets the design intensity duration
- Manning's equation — size the conveyance for the peak flow
- AquaSWMM rational-method sewer design — C by subcatchment, accumulated down the network