Gutter Downspout Calculator
Size downspouts for roof area
sq ft
in/hr
Formula
Flow = Roof Area × Rainfall / 96.23
Each downspout handles a maximum flow rate.
Examples
1500 sq ft roof, 4 in/hr
2-3 downspouts needed.
Frequently Asked Questions
About Gutter Downspout
Size downspouts for your roof drainage area.