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Gutter Downspout Sizing Calculator

Required number of 2"×3" or 3"×4" downspouts.

Estimate the number of 2"×3" (handles ~600 ft² roof area) or 3"×4" (~1200 ft²) downspouts needed for a roof, based on the roof area drained and the design rainfall intensity (in/hr).

Marcus Laurent By Marcus Laurent
Published Last reviewed 1 min read

Inputs

ft²
in/h

Results

Enter values and click Calculate to see results.
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How to use this calculator

  1. Fill in the inputs above using the units you already have.
  2. Values update automatically as you type — no submit button needed.
  3. Hover any result row for the underlying formula and intermediate values.

Formula

Capacity factor ∝ rainfall · area. # 2x3 = ceil(area · rainfall / 600). # 3x4 = ceil(area · rainfall / 1200).

In depth

Estimate the number of 2"×3" (handles ~600 ft² roof area) or 3"×4" (~1200 ft²) downspouts needed for a roof, based on the roof area drained and the design rainfall intensity (in/hr).