Downspout Size Calculator
Estimate roof runoff flow and a planning round-downspout diameter from catchment area, rainfall intensity, downspout count, design velocity and usable flow-area factor.
Default project preview
2 in nominal
Total roof runoff
83.1 gal/min
Flow per downspout
20.78 gal/min
Calculated inside diameter
1.56 in
Required flow area
1.9 in²
Calculator
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Enter the project details and calculate a rounded material estimate.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Roof runoff flow = catchment area × rainfall intensity × runoff coefficient. Flow is divided among downspouts. Required pipe area = flow per downspout ÷ (velocity × usable-area factor), then converted to a round diameter.
FAQ
Downspout Size Calculator questions
Where should rainfall intensity come from?+
Use project-specific local design rainfall data rather than a general annual rainfall value.
Why include a usable flow-area factor?+
It reduces the ideal full-pipe area to allow for non-full flow, turbulence and practical uncertainty.
Does a rectangular downspout use the same diameter?+
No. Compare the required flow area with the clear inside area and manufacturer capacity of the rectangular product.