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Downspout & Gutter Sizing Calculator

SMACNA Downspout & Gutter Calculator Terms of Use (required)

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Calculator Notes and Assumptions

  1. Calculator is based on Chapter 1 of SMACNA's Architectural Sheet Metal Manual 7th Edition (ASMM).
  2. 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.
  3. Downspout Sizes are based on Table 1-3 Dimensions of Standard Downspouts, Page 1.4 of the Architectural Sheet Metal Manual 7th Edition.
  4. Calculations are based on level gutter capacity as experimentally determined by the National Institute of Standards and Technology (NIST).
  5. Downspout Sizes must not exceed the bottom width of the gutter.
  6. 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.
  7. 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.