Stormwater Rational Method
Peak stormwater runoff from a drainage area, in cfs and gpm.
Example
You enter
- Catchment area (ft²) 5000
- Surface asphalt
- Rainfall intensity (in/hr) 2
You get
- Runoff coefficient 0.95
- Peak flow (cfs) 0.218
- Peak flow (GPM) 97.9
- Area (acres) 0.1148
Checked against published worked examples from: USEPA / NRCS.
Details, formula, and sources
ScopeQ = C * i * A peak runoff in cfs and gpm with bundled C values.
Formula or tableQ = C × i × A. C from bundled runoff coefficients per surface; i from user-supplied design rainfall intensity; A in acres yields Q in cfs (1 cfs ≈ 449 gpm).
Source and editionRational method by name (public hydrology). C-values from public engineering practice.
Free accessRainfall intensity / IDF curves free at hdsc.nws.noaa.gov (NOAA Atlas 14).
What governsEstimate. AHJ and licensed plumber / gas fitter govern. Verify against the IPC / IFGC edition adopted in your jurisdiction.
API field namesarea_ft2, surface, rainfall_in_per_hr, runoff_coefficient, peak_flow_cfs, peak_flow_gpm, area_acres
- C-values data/plumbing/runoff-coefficients.json keyed to surfacepublic engineering practice
- Time of concentration user-supplied; method assumes Q peaks at Tcrational method assumption