Rational Method Calculator

Calculate peak stormwater discharge using the Rational Method (Q = CiA). Free online calculator for drainage engineers and civil engineering professionals.

Calculate Peak Discharge

For educational purposes only. Not a substitute for professional engineering judgment.

Input Parameters

Dimensionless value from 0 to 1 based on surface type

Don't know this value? Look it upfor Runoff Coefficient (C)
in/hr

Design storm intensity for time of concentration

Don't know this value? Look it upfor Rainfall Intensity (i)
acres

Contributing area (max recommended: 200 acres)

years

Design storm frequency (2, 5, 10, 25, 50, or 100 years)

min

Time for runoff to reach outlet (for reference)

Rational Method Overview

The Rational Method estimates peak runoff rate using the equation Q = C × i × A where:

  • Q = Peak discharge (cfs)
  • C = Runoff coefficient (0-1, dimensionless)
  • i = Rainfall intensity (in/hr)
  • A = Drainage area (acres)

This method is widely used for small watersheds and assumes that peak flow occurs when the entire drainage area is contributing runoff.

Typical Runoff Coefficients

Surface TypeC (Low)C (High)
Asphalt/Concrete0.700.95
Roofs (impervious)0.750.95
Lawns (sandy soil)0.050.20
Lawns (clay soil)0.150.35
Parks/Cemeteries0.100.25
Industrial Areas0.500.90
Residential (1/4 acre lots)0.300.50

Source: FHWA HEC-22 (2009), Table 3-1. Use weighted average for mixed land uses.
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