written by NDS, Inc.

By following the steps outlined in this article, you will be able to determine the amount of water draining from your property and ways that this water can be efficiently stored.

Step 1: Calculate Stormwater Runoff using the Rational Method: Q = C x I x A where:

  • Q = Storm Water Runoff (in cubic feet per second)
  • C = Coefficient of Runoff
  • I = Rainfall Intensity (in inches per hour)
  • A = Area of Drainage Zone (in acres)
  • Draw a sketch of your home with the area measurements
  • Examine your property to determine where and how much water is draining.
Drainage Formula

Rational Method to Calculate Drainage

Step 2:  Calculate the volume of water being stored

  • Each municipality has their specific rules when it comes to determining how much water can be drained off of your property. A method that is commonly used is called the First Flush rule.
  • The First Flush rule states that the first 15 minutes (or other duration determined by your municipality) of water from a storm must be stored on your property. Any water after that first 15 minutes can be discharged off your property.

    Drainage Calculation

    Drainage Area in Square Feet

Step 3: Determine how to store runoff

  • There are two ways that this water can be stored:
    • Above ground in retention and detention ponds
    • Below ground in dry wells, french drains, etc.
  • The NDS Flo-Well and Ezflow are two products that can help you efficiently store your runoff.

Click here for a complete step by step guide on How to Calculate Drainage.

In a six part video series, NDS’ resident civil engineer, Dr. Drainage, walks you through how to calculate runoff, store runoff, size your catch basin and determine what drain pipe to use. Click here to view the entire series.

In addition to these videos, NDS has created a series of online calculators that make it easy for contractors to plug in information from their job site and it helps you calculate drainage for your dry well, French drain, and drain pipe.

NDS website: