Downspout Size Calculator
Calculate the correct downspout size for your gutter system based on roof area, rainfall intensity, and local climate conditions. Prevent water damage with proper drainage calculations.
Step 1: Roof Measurements
Typical values: 2-3 in/hr (light rain), 4-5 in/hr (moderate), 6+ in/hr (heavy rain)
Understanding Rainfall Intensity
Rainfall intensity varies by region. Check local building codes or historical weather data for your area’s maximum expected rainfall rate.
Step 2: Downspout Configuration
Downspout Sizing Guidelines
Standard downspout sizes: 2×3 inches (small), 3×4 inches (medium), 4×5 inches (large). Round downspouts: 2″, 3″, 4″, 5″ or 6″ diameter.
Downspout Size Results
Recommended Downspout Size
Downspout Size Capacity Comparison
| Downspout Size | Shape | Max Capacity (GPM) | Recommended Roof Area |
|---|
Flow Capacity Breakdown
Downspout Sizing Formula
Flow Rate (GPM) = Roof Area × Rainfall Intensity × Roof Pitch Factor ÷ 96.3
To calculate required downspout size: Multiply roof area by rainfall intensity and pitch factor, divide by 96.3 to get GPM, then select downspout size that meets or exceeds this flow rate with a safety margin.
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