Rational Method Runoff Calculator
Estimate peak stormwater runoff for small drainage areas using the Rational Method equation.
Rational Method runoff inputs and peak runoff output.
Rational Method Runoff Calculator
Estimate peak runoff using Q = C i A.
Use only where the Rational Method is accepted by the local agency.
What this calculator does
The Rational Method estimates peak runoff with Q = C i A, where Q is peak runoff, C is runoff coefficient, i is rainfall intensity, and A is drainage area.
Inputs explained
- Runoff coefficient C: Represents the portion of rainfall that becomes runoff.
- Rainfall intensity i: Rainfall intensity for the design storm and duration equal to the time of concentration.
- Drainage area A: Area draining to the point of interest, entered in acres.
Limitations
This is a planning and screening calculator. Confirm time of concentration, design storm, agency criteria, and allowable drainage area limits before using results for design.
FAQ
When is the Rational Method typically used?
It is commonly used for smaller drainage areas where the local stormwater or drainage manual allows it.
What units does the calculator use?
The calculator uses U.S. customary Rational Method units: Q in cfs, rainfall intensity in inches per hour, and area in acres.
Does the calculator select a runoff coefficient for me?
No. The user should select a runoff coefficient based on land cover, imperviousness, and local agency guidance.
Related tools
Need help reviewing this result?
Use this calculator for planning-level checks, then request help if you need a second look at assumptions, units, stormwater criteria, pump sizing, storage volume, or report-ready calculations.