Downspout Drainage Capacity Calculator
Find the peak roof runoff and how many downspouts you need, sized to your roof area, local rainfall intensity and downspout type.
How it works
Adjusted capacity = base area × (4 ÷ rainfall) · Downspouts = ceil(roof area ÷ adjusted capacity) · Peak flow = area × rainfall × 0.010383 gpm
A downspout can only carry so much water, so the number you need rises with roof area and with rainfall intensity. Each downspout type has a drainage area it handles at a reference intensity of about 4 inches per hour; in wetter climates that capacity shrinks in proportion to the higher rainfall. Peak runoff is figured from the fact that one inch of rain on one square foot is about 0.623 gallons, which works out to 0.0104 gallons per minute per square foot per inch/hour of intensity.
Worked example
An 1,800 sq ft roof at 4 in/hr sheds 1,800 × 4 × 0.010383 ≈ 74.8 gpm. A 3"×4" downspout handles about 1,200 sq ft at that intensity, so 1,800 ÷ 1,200 = 1.5 → 2 downspouts, each carrying roughly 37 gpm.
Frequently asked questions
What rainfall intensity should I use?
Use your local design intensity — the 5-minute rainfall for a 100-year storm from a regional rainfall table or plumbing code appendix. Common design values range from about 3 in/hr in dry regions to 7+ in/hr along the Gulf Coast.
Why does higher rainfall need more downspouts?
A downspout’s drainage-area rating assumes a reference intensity of roughly 4 in/hr. Double the rainfall and each downspout can only serve half the area, so heavy-rain regions need downspouts spaced more closely.
How far apart should downspouts be?
Beyond total capacity, gutters drain best with a downspout roughly every 30–40 feet so water does not have to travel too far. Use the count here as a minimum and add outlets on long runs.
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This calculator is for educational purposes. Double-check important results before acting on them.