Storm Water Runoff Calculator
Calculate peak runoff flow using the Rational Method for storm water design
About this calculator
The Storm Water Runoff Calculator uses the Rational Method to determine peak runoff flow rates for storm water management and drainage design. This essential tool helps engineers, urban planners, and developers calculate how much water will flow from a given area during rainfall events. By inputting rainfall intensity, drainage area size, and runoff coefficient values, users can accurately estimate peak discharge rates needed for designing adequate storm drainage systems, detention ponds, and flood control infrastructure.
How to use
Enter the rainfall intensity (inches per hour), drainage area (acres), and runoff coefficient for your surface type. The runoff coefficient varies from 0.1 for natural areas to 0.9 for paved surfaces. Click calculate to get the peak runoff flow rate in cubic feet per second using the Rational Method formula Q = CIA.
Frequently asked questions
What is the Rational Method formula?
The Rational Method uses Q = CIA, where Q is peak discharge, C is runoff coefficient, I is rainfall intensity, and A is drainage area.
How do I choose the right runoff coefficient?
Runoff coefficients range from 0.1-0.3 for natural areas, 0.4-0.6 for residential areas, and 0.7-0.9 for commercial or paved surfaces.
What rainfall intensity should I use?
Use the design storm intensity for your location and desired return period, typically found in local meteorological data or engineering standards.