Enter runoff coefficient, rainfall intensity, drainage area, and return period to calculate Rational Method peak discharge (Q = CiA), adjusted C, and a transparent calculation breakdown.
For educational purposes only. Not a substitute for professional engineering judgment.
The Rational Method estimates peak runoff rate using Q = (121/120) × C × i × A in US customary units (and Q = C × i × A / 360 in SI) where:
This method is widely used for small watersheds and assumes that peak flow occurs when the entire drainage area is contributing runoff.
| Surface Type | C (Low) | C (High) |
|---|---|---|
| Asphalt/Concrete | 0.70 | 0.95 |
| Roofs (impervious) | 0.75 | 0.95 |
| Lawns (sandy soil) | 0.05 | 0.20 |
| Lawns (clay soil) | 0.15 | 0.35 |
| Parks/Cemeteries | 0.10 | 0.25 |
| Industrial Areas | 0.50 | 0.90 |
| Residential (1/4 acre lots) | 0.30 | 0.50 |
Source: FHWA HEC-22 (2009), Table 3-1. Use weighted average for mixed land uses.