Rational Method Runoff Calculator
Size pipes, inlets, and small channels: pick a runoff coefficient for the land use, enter the design rainfall intensity and drainage area, and get the peak runoff rate.
Peak Runoff (Rational Method)
| Item | Value |
|---|
How the calculation works
Rational formula
Q (cfs) = 1.008 × C × I (in/hr) × A (acres)
1 acre-in/hr = 43,560 ft² × (1/12 ft) ÷ 3,600 s = 1.008 cfs — the 1.008 keeps the units exact (many references round it to 1.0).
Q (gpm) = Q (cfs) × 448.831
Q (cfs) = 1.008 × C × I (in/hr) × A (acres)
1 acre-in/hr = 43,560 ft² × (1/12 ft) ÷ 3,600 s = 1.008 cfs — the 1.008 keeps the units exact (many references round it to 1.0).
Q (gpm) = Q (cfs) × 448.831
| Land use | Typical C |
|---|---|
| Asphalt / concrete pavement | 0.70–0.95 |
| Commercial, downtown | 0.70–0.95 |
| Industrial | 0.50–0.90 |
| Multi-family residential | 0.40–0.75 |
| Single-family residential | 0.25–0.55 |
| Lawns, clay soil (flat) | 0.10–0.25 |
| Lawns, sandy soil (flat) | 0.05–0.15 |
| Parks and cemeteries | 0.10–0.30 |
Notes & limitations
The Rational Method and the coefficient ranges above follow FHWA Hydraulic Engineering Circular No. 22 (HEC-22), Urban Drainage Design Manual, and standard state DOT drainage manuals. Verify the C values and IDF data your reviewing agency requires.
- Use for small watersheds only — commonly limited to about 200 acres or less, with a single land use character.
- Assumes rainfall intensity is uniform over the whole watershed for a duration equal to the time of concentration; it gives a peak rate, not a hydrograph or a volume.
- Do not use it for detention/retention storage routing or for watersheds with significant storage — use a hydrograph method (e.g., SCS TR-55) instead.
- For mixed land uses, compute a weighted C = Σ(Ci × Ai) / ΣAi and enter it as a custom value.
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