Plumbing Calculator
Roof Drainage Calculator
Use our free roof drainage calculator to accurately determine the rainfall runoff volume from your roof, and properly size your gutters and downspouts based on your local rainfall intensity.
Table of Contents
Parameters
Results
Based on your input parameters
Roof Drainage Formula
Q = (A × I) / 96.23
Q = Runoff Flow Rate in Gallons per Minute (GPM).
A = Total Roof Area in square feet.
I = Rainfall Intensity in inches per hour.
96.23 = Conversion factor to convert area and intensity into GPM.
Downspout Sizing Table
| Downspout Diameter | Max Capacity (GPM) |
|---|---|
| 2 inches | 23 GPM |
| 3 inches | 54 GPM |
| 4 inches | 106 GPM |
| 5 inches | 188 GPM |
| 6 inches | 304 GPM |