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Rational Method Peak Runoff Calculator

Calculate preliminary peak runoff using Q = C × i × A with SI/US input support and a canonical SI engine.

Preliminary peak-runoff calculation only. Select rainfall intensity and runoff coefficient from an appropriate design basis.
01

Runoff inputs

Three engineering inputs. No hidden defaults.

Scenario
Catchment and design rainfall

Use rainfall intensity for the selected design event and an appropriate duration, commonly informed by the catchment time of concentration.

02

Peak runoff result

Enter the design inputs and calculate.

Ready for calculation

The result will show peak runoff and the canonical calculation inputs.

Methodology

Rational Method peak runoff

TOOL-003 calculates peak runoff with Q = C × i × A. Internally, rainfall intensity is converted to m/s and catchment area to m², so the canonical result is m³/s.

Input guidance

Applicability and limitations

The Rational Method is intended for peak-flow estimation under its accepted assumptions. This calculator does not derive an IDF curve, time of concentration, hydrograph, detention routing, pipe capacity, or network hydraulics. It does not automatically establish code or permit compliance.

Worked example

For 10 ha, C = 0.80 and 50 mm/h, the canonical result is approximately 1.111111 m³/s.

Source

Method reference: FHWA HEC-22 Urban Drainage Design Manual, represented in the WaterEngineerLab Source Registry as SRC-RATIONAL-METHOD-TOOL003-001.