DrainageCalculators

Rational Method Calculator

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

What This Solves

Estimates peak stormwater runoff rate from a small drainage area using the formula Q = C * i * A.

Best Used When

  • The drainage area is under 200 acres (80 hectares)
  • You need a quick peak flow estimate for inlet, pipe, or ditch sizing
  • The land use across the drainage area is fairly uniform

Do NOT Use When

Key Assumptions

  • Rainfall intensity is uniform over the entire drainage area
  • Storm duration equals the time of concentration
  • The runoff coefficient remains constant during the storm
  • Peak flow occurs when the entire area is contributing runoff to the outlet

Input Quality Notes

Accuracy depends heavily on a good estimate of the runoff coefficient (C) and rainfall intensity. Use local IDF curves for intensity and verify C with field conditions when possible.

Try a Common Scenario

Click to pre-fill the calculator with realistic values.

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.

Professional Review: For projects requiring regulatory approval or where public safety is involved, have your calculations reviewed and stamped by a licensed Professional Engineer (PE).

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Last verified: February 2026

Disclaimer & Terms of Use

Educational Purpose: The calculations provided by DrainageCalculators.com are for informational and educational purposes only. While we strive for accuracy, these tools do not substitute for professional engineering judgment or designs stamped by a licensed Professional Engineer (PE) where required by law.

Professional Certification: Users are solely responsible for verifying all calculations, confirming compliance with local codes and regulations, and determining the appropriateness of any methodology for their specific application. Critical infrastructure and public safety projects require review and certification by a licensed Professional Engineer.

Limitation of Liability: DrainageCalculators.com, its owners, operators, and contributors assume no liability for damages or losses arising from the use of these calculators. By using this tool, you acknowledge that you have read, understood, and agreed to these terms.

PE Stamp Required: Many jurisdictions require drainage designs to be reviewed and stamped by a licensed Professional Engineer. Check your local requirements before using these calculations for construction or permit applications.