National Stormwater Calculator
Help control runoff and promote the natural movement of water
EPA’s National Stormwater Calculator (SWC) is a software application tool that estimates the annual amount of rainwater and frequency of runoff from a specific site using green infrastructure as low impact development controls. The SWC is designed for use by anyone interested in reducing runoff from a property, including site developers, landscape architects, urban planners, and homeowners.
Software, Compatibility, and Manuals
The SWC is available as a mobile web-based application or as a desktop program—both versions require an internet connection. The mobile web-based app can be used on desktop devices and mobile devices, such as smartphones and tablets, and is compatible with all operating systems. The Windows-based desktop program runs on any version of Microsoft Windows with Version 4 or higher of the .Net Framework installed.
Both versions work best with the following web browsers: Microsoft Edge, Google Chrome, Mozilla Firefox, and Apple Safari.
|08/10/2022||National Stormwater Calculator Mobile Web-Based App (Version 3.4.0)|
National Stormwater Calculator Version 18.104.22.168 (ZIP) (zip)
|08/10/2022||National Stormwater Calculator User's Guide—Mobile Web-Based Application Version 3.4.0|
|08/01/2019||National Stormwater Calculator User's Guide—Desktop Version 22.214.171.124|
|08/01/2019||National Stormwater Calculator Desktop Version|
Disclaimer: Any mention of trade names, manufacturers, or products does not imply an endorsement by EPA. EPA and its employees do not endorse commercial products, services, or enterprises.
The SWC uses the Storm Water Management Model (SWMM) as its computational engine. SWMM is a well-established, EPA developed model that has seen continuous use and periodic updates for 40 years. Its hydrology component uses physically meaningful parameters making it especially well-suited for application on a nation-wide scale. SWMM is set up and run in the background without requiring any involvement of the user. The SWC accesses several national databases that provide soil, topography, rainfall, and evaporation information for a chosen site.
The SWC allows users to analyze site hydrology for small- to medium-sized (less than 12 acres) locations within the United States, including Puerto Rico, using LID controls. It estimates the amount of stormwater runoff generated from a site under different development and control scenarios over a long-term period of historical rainfall.
An LID cost estimation module within the application allows planners and managers to evaluate LID controls based on comparison of regional and national project planning level cost estimates (capital and average annual maintenance) and predicted LID control performance. Cost estimation is accomplished based on user-identified size configuration of the LID control infrastructure and other key project and site-specific variables. This includes whether the project is being applied as part of new development or redevelopment and if there are existing site constraints.
The SWC allows users to consider how runoff may vary based on historical weather and potential future climate conditions. To better inform decisions, it is recommended that users develop a range of results with various assumptions about model inputs. Please check with local authorities about whether and how use of these tools may support local stormwater management goals.
Green Infrastructure as LID Controls
Green infrastructure practices, which are the LID controls used in the SWC, promote the natural movement of water, instead of allowing it to wash into streets and down storm drains. Having less water runoff into storm drains and roadways can help prevent contamination of waterways, infrastructure degradation, flooding, and overwhelming of treatment plants. This allows stormwater to be used as a resource rather than a waste product, and can add aesthetic and economic value to a community. The following green infrastructure practices are included in the SWC:
- Rooftop (Downspout) Disconnection. This practice allows rooftop rainwater to discharge to pervious landscaped areas and lawns instead of directly into storm drains. It can be used to store stormwater and/or allow stormwater to infiltrate into the soil.
- Rainwater Harvesting (Rain Barrels or Cisterns). Containers that collect roof runoff collect runoff from rooftops and convey it to a tank where it can be used for non-potable water uses and onsite infiltration. Cisterns may be located above or below ground and have a greater storage capacity than rain barrels.
- Rain Gardens. Hallow depressions filled with an engineered soil mix that supports vegetative growth. They provide opportunity to store and infiltrate captured runoff and retain water for plant uptake. They are commonly used on individual home lots to capture roof runoff.
- Green Roofs (also known as vegetated roofs). Bioretention systems placed on roof surfaces that capture and temporarily store rainwater in a soil medium. They consist of a layered system of roofing designed to support plant growth and retain water for plant uptake while preventing ponding on the roof surface.
- Street Planters. Consist of concrete boxes filled with an engineered soil that supports vegetative growth and are typically placed along sidewalks or parking areas. Beneath the soil is a gravel bed that provides additional storage as the captured runoff infiltrates into the existing soil below.
- Infiltration Basins. Shallow depressions filled with grass or other natural vegetation that capture runoff from adjoining areas and allow it to infiltrate into the soil. They provide storage volume and additional time for captured runoff to infiltrate the native soil below.
- Porous Pavement. Excavated areas filled with gravel and paved over with a porous concrete or asphalt mix or with modular porous blocks. Normally all rainfall will immediately pass through the pavement into the gravel storage layer below it where it can infiltrate at natural rates into the site's native soil.
The SWC is most appropriate for performing screening level analysis of small footprint sites up to several dozen acres in size with uniform soil conditions. Its primary focus is informing site developers and property owners on how well they can meet a desired stormwater retention target. It can be used to answer questions such as the following:
- What is the largest daily rainfall amount that can be captured by a site in predevelopment, current, or post-development condition?
- To what degree will rainfall from storms of different magnitudes be captured on site?
- What mix of LID controls can be deployed to meet a given stormwater retention target?
- How well will LID controls perform under future meteorological projections made by global climate change models?
- What are the relative planning level costs (capital and maintenance) differences for various mixes of LID controls?
Resources and Technical Support
Publications and Other Material:
- Frequently Asked Questions
- Additional comments or questions: email us at SWC@EPA.gov.
- Green Infrastructure Modeling Toolkit can be used as a teaching tool and as a quick reference resource for use by planners and developers when making green infrastructure implementation decisions for stormwater management.
- Stormwater Management and Green Infrastructure Research website provides information on EPA's research efforts.
- Green Infrastructure website provides basic information, useful tools, research, case studies, and a publication library.
- Low Impact Development website provides information and guidance on the use of urban runoff LID controls.