Stormwater Treatment Practice Calculator

The STP Calculator is a tool developed by the Department of Environmental Conservation (DEC) to estimate total phosphorus load reductions achieved by stormwater treatment practices (STPs). The user enters STP data into the tool and the tool calculates the estimated annual average total phosphorus load reduction. Calculations are based on the same methods DEC will use to track progress reducing phosphorus pollution loading into Lake Champlain and Lake Memphremagog. The tool can currently only be applied to estimate total phosphorus reductions in the Lake Champlain and Lake Memphremagog watersheds, as pollutant loading rates are currently unavailable outside these basins in Vermont. The calculator tool focuses on STPs treating runoff from developed lands, only, and should not be used for agricultural, forested, or other types of land use. The calculator may not be suited for complex STP systems or retrofit projects. The STP calculator should only be used for planning purposes to understand pollutant reduction potential for STPs. Data entered in the STP calculator will not be stored in a database. DEC retains the right to verify the data input and will provide final phosphorus load reduction crediting based on data reported to and stored in DEC’s Watershed Projects Database.

Instructions can be found Here.

Please direct any questions to or 802-636-7536.

STP Calculator

Loading Information


STP Information


Estimated Phosphorus Reduction



  • 11/30/2018 - Please note "Sand Filter w/ underdrain" efficiencies have been updated based on input from EPA.