The Rational Method is the simplest and most widely used way to estimate peak runoff from small drainage areas, for sizing inlets, pipes, swales and temporary construction controls.
The formula
Q = C × i × A
- Q = peak runoff rate, in cubic feet per second (cfs)
- C = runoff coefficient, the fraction of rainfall that becomes runoff (0 to 1)
- i = rainfall intensity, in inches per hour, for the design storm at a duration equal to the time of concentration
- A = drainage area, in acres
The units work out because 1 acre-inch per hour is about 1.008 cfs, so the factor is dropped by convention.
Typical runoff coefficients
| Surface | Typical C |
|---|---|
| Roofs | 0.75 to 0.95 |
| Asphalt and concrete pavement | 0.70 to 0.95 |
| Commercial / downtown areas | 0.70 to 0.95 |
| Single-family residential | 0.30 to 0.50 |
| Lawns, sandy soil, flat to average slope | 0.05 to 0.15 |
| Lawns, clay soil, flat to average slope | 0.13 to 0.22 |
| Lawns, clay soil, steep | 0.25 to 0.35 |
| Woods | about 0.05 to 0.25 |
Values vary by manual and storm frequency; use your local stormwater manual's table.
Weighted (composite) C
For mixed land cover: C = (C1 × A1 + C2 × A2 + ...) ÷ total area.
Where intensity comes from
Look up rainfall intensity in NOAA Atlas 14 precipitation frequency data (or your local IDF curves) for the design return period, such as the 10-year storm, at a duration equal to the time of concentration.
Limitations
The Rational Method assumes uniform rainfall over the whole area and gives a peak rate, not a volume or hydrograph. Many manuals limit it to drainage areas of roughly 100 to 200 acres or less. For detention design and larger areas, engineers use hydrograph methods such as NRCS (SCS) TR-55.
Sources and official references
Last reviewed: October 25, 2026 · Free general education based on the Clean Water Act, EPA NPDES rules and the EPA Construction General Permit. Your governing permit (EPA, state, tribal or local) controls. This is not legal advice, not EPA-approved, and not a California QSP/QSD or other state credential, and it does not issue a certificate. Nothing you enter on this page is saved or sent.