Downspout & Gutter Sizing Calculator

This app allows you to easily and accurately size downspout and gutters using the methods and formulas covered in SMACNA’s Architectural Sheet Metal Manual (ASMM).

The Rainfall Wizard allows you to select the closet city to use the information from the ASMM tables. The Rain Intensity per ASMM Table 1-2 uses the 5-minute Duration for a 10yr or 100yr storm. Since the Rain Intensity data is constantly being updated by the National Oceanic and Atmospheric Administration (NOAA), this calculator also app allows the manual entry of the Rainfall Intensity, thereby allowing you to calculate per the local authority having jurisdiction. For the most up to date rain intensity data, NOAA’s website is (Use Precipitation Intensity as your data type.)

Either manually enter the Design Area of the roof or use the wizard. The Design Area Wizard allows the user to select a roof shape and add dimensions to get a design area. The wizard also calculates for Vertical Walls area per International Plumbing Code (IPC.)

The required gutter and downspout size is dependent on the number of sections, the length of gutter sections, the number of downspouts, and the gutter width to height ratio.

Downspout and Gutter Sizing Calculator App