DRAINAGE CAPACITY SIZING
Downspout Size Calculator
Determine minimum downspout quantity and sizing capacity (2×3" vs. 3×4") needed to handle roof runoff flow.
Downspout Size Calculator
Determine minimum downspout quantity and size capacity (2×3" vs. 3×4") needed to handle roof runoff.
GPM
outlets
Downspout Sizing Formula
$$text{Min Downspouts} = leftlceil frac{text{Peak Runoff Flow (GPM)}}{text{Rated Downspout Capacity (GPM)}} rightrceil$$
Standard 2″ × 3″ downspouts handle ~30 GPM (~600 sq ft @ 3 in/hr), while commercial 3″ × 4″ downspouts handle ~60 GPM (~1,200 sq ft).