Stormwater Runoff Calculator

This calculator allows you to simulate the effects that installing a rain garden,  a type of green infrastructure, could have on stormwater runoff from your property.  The calculator has two primary functions:

The first is that It will size a rain garden for you that treats 1" of rainfall.  This rainfall depth is often recommended for sizing GI structures because it will treat ~90% of all of rainfall runoff that occurs during a year of average precipitation.

Also, it will introduce you to probabilisitc rain events. These are rain storms that are predicted to occur every so often (in this case 10 years) and that last for a given amount of time (here 24 hours). The longer the return interval, and longer the duration, the greater the storm depth.

This second function of the website allows you to specify the size of a rain garden you plan to install.  Based on the GI area you specify, it will approximate the volume of rainfall runoff that will be generated on your property by a 24 hour long design rain event, that has the probability of occurring once every ten years.

To do this, you will be directed through 3 separate web pages. Each of these pages will instruct you how to gather important physical data that describes your property. The sources for this data include publically available servers and interactive maps.

The information you will gather is:

  1. How well the soils on your property infiltrate rain water
  2. The amount of impervious area and the potential runoff factor of your site
  3. The amount of rainfall in the event of a 24 hour, 10 year storm event.

Also, you will be able to designate part of your property as a site for installing a rain garden. The much space on your property you can allocate to GI, the less runoff your property will generate.


The model used to determine the amount of runoff is the National Resource Conservation Service's Technical Release 55. For more details on this runoff model click on here for the Model Description.



Note: For the calculator to work, you may need to allow ActiveX content.