Calculator Notes and Assumptions
- Calculator is based on Chapter 1 of SMACNA's Architectural Sheet Metal Manual 7th Edition (ASMM).
- Rainfall Wizard is based on Table 1-2 Rainfall Data and Drainage Factors, Page 1.2-1.3 of the Architectural Sheet Metal Manual 7th Edition.
- Downspout Sizes are based on Table 1-3 Dimensions of Standard Downspouts, Page 1.4 of the Architectural Sheet Metal Manual 7th Edition.
- Calculations are based on level gutter capacity as experimentally determined by the National Institute of Standards and Technology (NIST).
- Downspout Sizes must not exceed the bottom width of the gutter.
- To limit the effects of thermal expansion in gutters, 50 ft. (15 m) is a practical maximum gutter length to be served by a downspout.
- The capacity of a sloped rectangular gutter may be approximated by using a gutter cross section area not less than that of a semicircular gutter and a depth to width ratio of at least 0.75.