Rational Method Runoff Coefficient (C) — Reference
For peak runoff: Q = C·i·A (US units — Q in cfs, i in in/hr, A in acres). The Rational Method is appropriate for drainage areas under ~200 acres, where the rainfall intensity is reasonably uniform and the hydrograph shape isn't needed. For larger watersheds or when you need volume / hydrograph routing, switch to the NRCS curve-number method.
By Surface Type
| Surface | C (typical) | Range |
|---|---|---|
| Asphalt or concrete pavement | 0.90 | 0.85–0.95 |
| Brick / unit pavers, sealed joints | 0.80 | 0.70–0.85 |
| Roof, metal or membrane | 0.95 | 0.90–0.95 |
| Gravel, well-compacted | 0.55 | 0.40–0.65 |
| Gravel, loose | 0.35 | 0.20–0.50 |
| Bare clay soil, packed | 0.55 | 0.40–0.65 |
| Bare sandy soil, loose | 0.20 | 0.10–0.30 |
By Land Use
| Land use | C (typical) | Range |
|---|---|---|
| Downtown business district | 0.85 | 0.70–0.95 |
| Neighborhood business district | 0.65 | 0.50–0.70 |
| Light industrial | 0.60 | 0.50–0.80 |
| Heavy industrial | 0.75 | 0.60–0.90 |
| Residential, multifamily attached | 0.65 | 0.60–0.75 |
| Residential, multifamily detached | 0.50 | 0.40–0.60 |
| Residential, single-family (~¼ ac lots) | 0.40 | 0.35–0.50 |
| Residential, single-family (~½ ac lots) | 0.30 | 0.25–0.40 |
| Residential, large estate (1+ ac lots) | 0.25 | 0.20–0.35 |
| Parks, cemeteries | 0.20 | 0.10–0.30 |
| Playgrounds, school grounds | 0.30 | 0.20–0.40 |
| Railroad yards | 0.30 | 0.20–0.40 |
| Unimproved / open land | 0.20 | 0.10–0.30 |
By Lawn / Soil / Slope
| Lawn surface | Slope | C |
|---|---|---|
| Sandy soil, lawn | Flat (< 2%) | 0.05–0.10 |
| Avg (2–7%) | 0.10–0.15 | |
| Steep (> 7%) | 0.15–0.20 | |
| Heavy / clay soil, lawn | Flat (< 2%) | 0.13–0.17 |
| Avg (2–7%) | 0.18–0.22 | |
| Steep (> 7%) | 0.25–0.35 |
Composite (Weighted) C for a Mixed Site
For a site with multiple surfaces:
Ccomposite = Σ(Ci · Ai) / ΣAi
Worked example for a 1.5-ac developed lot with 0.4 ac roof, 0.5 ac asphalt, 0.6 ac lawn (clay, flat):
C = (0.95·0.4 + 0.90·0.5 + 0.15·0.6) / 1.5
C = (0.380 + 0.450 + 0.090) / 1.5 = 0.61
Sources: ASCE/WEF MOP-37 (Design and Construction of Urban Stormwater Management Systems), 1992. NRCS TR-55. Wright-McLaughlin Engineers, Urban Storm Drainage Criteria Manual (UDFCD).
Related cheat sheets and tools
Sizing the conveyance pipes after computing Q? See Manning's n for the gravity-flow run, then run culvert hydraulics at any major crossing. To get the "i" (rainfall intensity) for a specific Tc, use NOAA Atlas 14 IDF curves — PE-Calc's time-of-concentration tool gives the Tc, and the Rational Method tool plugs in i directly. For full storm-event simulation with hydrograph routing, ponds, and BMPs, see HydroComplete.