Downspout Calculator
Calculate downspout requirements, gutter drainage capacity, and materials needed for your roof. Get precise sizing and installation estimates.
Step 1: Roof Dimensions
Roof Area Calculation
Effective roof area considers roof pitch. Steeper roofs collect more rainwater per square foot of horizontal area.
Step 2: Downspout Details
Downspout Sizing
Proper downspout sizing prevents overflow and water damage. A 3×4 inch downspout can handle approximately 600 sq ft of roof area in moderate rainfall.
Downspout Calculation Results
Total Project Cost
Downspout Drainage Capacity
Capacity Guidelines
Downspout capacity varies by size and shape. Rectangular downspouts provide more capacity per inch than round ones of equivalent area. Always include a safety margin for heavy rainfall.
Downspout Size Comparison
Downspout Sizing Formula
Downspouts Needed = (Roof Area × Rainfall Intensity) / (Downspout Capacity × Safety Factor)
Calculate effective roof area considering pitch. Multiply by rainfall intensity for total water volume. Divide by individual downspout capacity with safety factor to determine number needed.
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