About this tool
The Gutter & Downspout Sizing Calculator determines the gutter size, number of downspouts and total linear feet needed to drain a roof. Enter the eave length and the horizontal roof depth draining to that gutter, choose the roof pitch, type in the local rainfall intensity in inches per hour, and pick the gutter profile and downspout size. All figures are computed in your browser using the SMACNA sizing method.
The roof's plan area is eave length × roof depth. A pitch factor increases the effective drainage area because steeper roofs catch wind-driven rain: flat to 3/12 = 1.0, 4–5/12 = 1.05, 6–8/12 = 1.10, 9–11/12 = 1.20 and 12/12 or steeper = 1.30. Because the capacity tables are published at 1"/hr, the effective area is scaled by your rainfall intensity: adjusted area = plan area × pitch factor × rainfall. The tool then picks the smallest gutter of your chosen profile whose 1"/hr capacity meets that adjusted area — a 5" K-style handles ~5,520 ft², a 6" ~7,960 ft², a 5" half-round ~2,500 ft² and a 6" ~3,840 ft².
Downspout count is the adjusted area divided by the per-downspout capacity (2×3" ≈ 600 ft², 3×4" ≈ 1,200 ft², 3" round ≈ 700 ft², 4" round ≈ 1,255 ft²), and never fewer than one downspout per 40 ft of gutter run. Total gutter linear feet equals the eave length you entered. Look up your rainfall intensity from a local 5-minute or 1-hour storm value for an accurate result.