Downspout Extension Calculator
Size downspout extensions for drainage
sq ft
in/hr
ft
Formula
Flow = Roof Area × Rainfall × 0.623 / 96
Calculate water flow from roof to size downspout extensions.
Examples
500 sq ft roof, 2 in/hr rain
~6.5 GPM flow rate.
Frequently Asked Questions
About Downspout Extension
Size downspout extensions for proper drainage away from foundations.