Storm Drain Sizing Calculator
Size roof drains, leaders, and storm piping
Horizontal Piping (if applicable)
Storm Drainage Results
Total Flow Rate
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Flow per Drain
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Roof Drain Size
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Leader/Vertical Pipe
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Horizontal Drain
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Note: Roof drain strainer area should be 1.5× to 2× the leader pipe area.
Leader/Vertical Drain Capacity
Square feet of roof area at 4 in/hr rainfall:
| Pipe Size | Sq Ft | GPM |
|---|---|---|
| 2" | 720 | 23 |
| 3" | 2,200 | 67 |
| 4" | 4,600 | 144 |
| 5" | 8,650 | 261 |
| 6" | 13,500 | 424 |
Horizontal Storm Drain Capacity (1/4" slope)
Square feet of roof area at various rainfall intensities:
| Pipe Size | 1 in/hr | 2 in/hr | 4 in/hr | 6 in/hr |
|---|---|---|---|---|
| 3" | 3,288 | 1,644 | 822 | 548 |
| 4" | 7,520 | 3,760 | 1,880 | 1,253 |
| 5" | 13,360 | 6,680 | 3,340 | 2,227 |
| 6" | 21,400 | 10,700 | 5,350 | 3,567 |
| 8" | 46,000 | 23,000 | 11,500 | 7,667 |