Stormwater Runoff Calculator
Calculate stormwater runoff volume and peak flow using rational method
About this calculator
The Stormwater Runoff Calculator uses the rational method to determine stormwater runoff volume and peak flow rates for drainage design and flood management. This essential tool helps engineers, urban planners, and developers calculate how much water will flow from a given area during rainfall events. By providing accurate runoff calculations, it enables proper sizing of storm drains, retention ponds, and other stormwater infrastructure to prevent flooding and meet regulatory requirements.
How to use
Enter the drainage area size, rainfall intensity, and runoff coefficient for your surface type (concrete, grass, etc.). The calculator applies the rational method formula Q=CiA to compute peak discharge flow rate. Results show both runoff volume and peak flow to help design appropriate stormwater management systems.
Frequently asked questions
What is the rational method in stormwater calculations?
The rational method uses the formula 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 vary by surface type: concrete (0.9), asphalt (0.85), grass (0.3), and forest (0.15). Mixed areas require weighted averages.
What rainfall intensity should I use?
Use local historical rainfall data for your desired storm frequency (10-year, 100-year) and duration matching your area's time of concentration.