Gutter calculator

Gutter sizing depends on roof area, pitch, and design rainfall intensity. Steeper roofs collect more water per square foot. The calculator uses industry-standard capacity tables: 5" K-style handles ~5,520 sq ft @ 1"/hr, 6" K-style handles ~7,960 sq ft @ 1"/hr. Capacity scales inversely with rainfall intensity.

Downspout spacing typically ranges from 20-40 feet depending on gutter size. Larger gutters can handle longer spans. Minimum 2 downspouts per building. The calculator determines optimal spacing based on roof area and gutter capacity.

Downspout size depends on roof area per downspout: 2×3" handles up to 600 sq ft, 3×4" handles up to 1,200 sq ft, 4×5" handles up to 2,400 sq ft. The calculator automatically selects appropriate size based on calculated area per downspout.

Steeper roofs increase effective drainage area because they collect more water per square foot. The calculator applies a pitch factor: Effective Area = Roof Area × √(1 + (pitch/12)²). A 6/12 pitch increases effective area by approximately 12% compared to a flat roof.

Design rainfall intensity varies by location. Common values: 2-3 in/hr for moderate climates, 3-4 in/hr for high rainfall regions, 4-5 in/hr for extreme climates. The calculator automatically looks up location-specific values from NOAA Atlas 14 data, or you can manually override.