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
Build the material order
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Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Roof catchment area
Enter this project assumption in ft². The current imperial default is 2000 ft².
Design rainfall intensity
Enter this project assumption in in/hr. The current imperial default is 4 in/hr.
Number of downspouts
Enter this project assumption in a unitless value. The current imperial default is 4. Minimum: 1.
Runoff coefficient
Enter this project assumption in a unitless value. The current imperial default is 1. Minimum: 0.1. Maximum: 1.
Planning water velocity
Enter this project assumption in ft/s. The current imperial default is 5 ft/s.
Usable flow-area factor
Enter this project assumption in %. The current imperial default is 70 %. Minimum: 10. Maximum: 100.
How to read the outputs
Total roof runoff
rainfall × area × runoff coefficient. Default preview: 83.1 gal/min.
Flow per downspout
equal-flow assumption. Default preview: 20.78 gal/min.
Calculated inside diameter
before nominal rounding. Default preview: 1.56 in.
Required flow area
velocity and usable-area assumption. Default preview: 1.9 in².
Rainfall intensity
user-entered design event. Default preview: 4 in/hr.
Usable flow area
planning capacity factor. Default preview: 70%.
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.