National Stormwater Calculator

EPA’s National Stormwater Calculator (SWC) is a desktop application that estimates the annual amount of rainwater and frequency of runoff from a specific site anywhere in the United States (including Puerto Rico). Estimates are based on local soil conditions, land cover, and historic rainfall records.

It is designed to be used by anyone interested in reducing runoff from a property, including

  • site developers,
  • landscape architects,
  • urban planners, and
  • homeowners.

The Calculator accesses several national databases that provide soil, topography, rainfall, and evaporation information for the chosen site. The user supplies information about the site’s land cover and selects the types of low impact development (LID) controls they would like to use. The LID controls that the user can choose are the following seven green infrastructure practices:

  1. Disconnection
  2. Rain harvesting
  3. Rain gardens
  4. Green roofs
  5. Street planters
  6. Infiltration basins
  7. Porous pavement

Green infrastructure promotes the natural movement of water, instead of allowing it to wash into streets and down storm drains. Green infrastructure also has the added benefit of beautifying neighborhoods and increasing property values.

The SWC estimates runoff at a site based on available information such as soil type, landscape and land-use information, and weather. This update to the SWC will allow users to consider how runoff may vary based on historical weather and potential future climate. To better inform decisions, it is recommended that the user develop a range of SWC results with various assumptions about model inputs such as percent of impervious surface, soil type, sizing of green infrastructure, as well as historical weather and future climate change scenarios. Please check with local authorities about whether and how use of these tools may support local stormwater management goals and requirements.

Clean water is essential to keeping our families and the environment healthy. The Calculator helps protect and restore the environmental integrity of our waterways.

To Install the Calculator on Your Computer:
  1. Download the National Stormwater Calculator Version (EXE) (17 MB)
  2. If your browser offers the option to run the setup program then do so.
  3. Otherwise have your browser display its list of recent downloads and select the setup file to run it.
  4. If you have problems installing the calculator contact your system administrator or try the alternative method described below.
Alternative Installation Method:
  1. Download National Stormwater Calculator Version (ZIP) (7 MB)  to your computer.
  2. Locate where your browser saved the zip file and move it to a folder you created just for the calculator.
  3. Unzip and right click on the file named StormwaterCalculator.exe.
  4. Select 'Create Shortcut'.
  5. Drag the shortcut onto your desktop and use it to launch the calculator.
  6. (Optional) Right-click the shortcut to rename it and/or move it to your Start menu.
To learn more about how to install the calculator, how to run it, and how to interpret its output:
Read the National Stormwater Calculator User's Guide Version 1.1 (PDF) (76 pp, 3.01 MB).

Additional Information

U.S. EPA. (2014) National Stormwater Calculator User's Guide Version 1.1 (PDF) (67 pp, 3.01 MB) Publication No. 600/R-13/085c.

Basic Fact Sheet: The Stormwater Calculator - Identifying Green Infrastructure Solutions

Technical Fact Sheet: U.S. EPA. (Revised 2014) National Stormwater Calculator - A desktop tool that helps users control runoff to promote the natural movement of water (PDF) (1 pp, 3.96 KB) Publication No. 600/F-13/095b.

EPA's National Stormwater Calculator (Video) (1:21 min, EPA YouTube)

Helpful Resources

  • Stormwater Management: provides information and guidance on stormwater management practices.
  • Low Impact Development: provides information and guidance on the use of low impact development (LID) practices.
  • Green Infrastructure: provides basic information, useful tools, as well as research, case studies and a publication library.
  • Climate Change: provides information and resources on climate change and its effects in the U.S. and globally


EPA National Stormwater Calculator Team

SWC version text file(1 pg, 7 B)