Gutter Calculator

Gutter sizing ensures roof runoff drains fast enough to prevent overflow, leaks, and structural water damage. The correct calculation depends on the roof catchment area, the pitch factor, and your local rainfall intensity.

Design rainfall flow rate: Q = A × R × C

Where:

• Q — peak flow rate (gallons per minute or liters per second)
• A — effective catchment area (sq ft or m²) — horizontal projection of roof, not actual surface area
• R — design rainfall intensity (in/hr or mm/hr) — use 100-year storm data for your region
• C — runoff coefficient (0.95 for standard pitched roofs; 1.0 for flat/metal roofs)

Pitch correction factor: Steep roofs shed water faster. SMACNA (Sheet Metal and Air Conditioning Contractors’ National Association) recommends multiplying catchment area by a pitch factor:

• Up to 3:12 pitch: factor 1.00
• 4:12 to 5:12: factor 1.05
• 6:12 to 8:12: factor 1.10
• 9:12 to 11:12: factor 1.20
• 12:12+: factor 1.30

Gutter capacity by size (half-round, 1 in/hr rain):

• 4-inch K-style: 5,520 sq ft catchment area
• 5-inch K-style: 7,960 sq ft
• 6-inch K-style: 11,520 sq ft
• 4-inch half-round: 5,000 sq ft
• 6-inch half-round: 11,750 sq ft

Downspout sizing rule of thumb:

• 1 downspout per 20–40 linear feet of gutter
• 2″ × 3″ rectangular downspout: handles ~600 sq ft roof area
• 3″ × 4″ rectangular downspout: handles ~1,200 sq ft roof area

Worked example: House footprint: 40 ft × 60 ft = 2,400 sq ft. Roof pitch: 6:12 (pitch factor 1.10). Design rainfall: 4 in/hr (heavy storm region).

• Adjusted area: 2,400 × 1.10 = 2,640 sq ft
• Flow rate: 2,640 × 4 ÷ 96.23 = approximately 110 GPM (using standard conversion)
• Required: 6-inch K-style gutter on the long sides, 5-inch on short sides
• Downspouts: at least 4 downspouts (one per corner), 3″ × 4″ minimum

Gutters should slope 1/4 inch per 10 feet toward downspouts to ensure self-draining flow.