Gutter Downspout Calculator
Estimate your roof's peak water flow and the number of downspouts needed to drain it.
Result
How to use
- Enter your values in the fields above.
- Press Calculate to see your result instantly.
- Use the Share button to copy a link to your result.
About this calculator
This calculator estimates peak roof runoff using the Rational Method, Q = C·i·A, where C is a runoff coefficient close to 1.0 for a roof surface, i is local rainfall intensity in inches or millimeters per hour, and A is the roof's plan area, then converts that flow into a downspout count using the rule of thumb that one square inch of downspout cross-section drains roughly 100 square feet of roof area at typical storm intensities.
Roofing contractors and gutter installers use this sizing to decide between 2×3 inch and 3×4 inch downspouts and to determine how many are needed on a given roof, since undersized or too-few downspouts cause gutters to overflow during heavy storms, leading to fascia rot and foundation water damage.
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