Size downspouts using roof load and local rainfall. Review capacity, velocity, and outlet recommendations instantly. Make drainage planning simpler with practical, code-friendly sizing guidance.
The page uses one overall column. The form itself uses a responsive grid with three columns on large screens, two on smaller screens, and one on mobile.
The chart compares required flow per downspout against the capacity of common outlet sizes using the selected design velocity.
| Scenario | Roof Length (ft) | Roof Width (ft) | Rainfall (in/hr) | Downspouts | Approx. Required Area (in²) | Suggested Size |
|---|---|---|---|---|---|---|
| Small Porch Roof | 20 | 15 | 2 | 2 | 6.00 | 2 x 3 |
| Single Family Roof Section | 50 | 30 | 4 | 4 | 6.00 | 2 x 3 |
| Large Commercial Section | 90 | 45 | 6 | 6 | 12.00 | 3 x 4 |
1) Plan roof area
Plan Area = Roof Length × Roof Width
2) Pitch factor
Pitch Factor = √(Run² + Rise²) ÷ Run
3) Effective roof area
Effective Area = Plan Area × Pitch Factor
4) Peak runoff by Rational Method
Q (cfs) = C × i × A(acres) × Safety Factor
5) Required downspout area
Required Area = Flow per Downspout ÷ Velocity
6) Equivalent round diameter
Diameter = √(4 × Area ÷ π)
This approach gives a practical sizing estimate. Final selection should still reflect local rainfall data, overflow strategy, gutter capacity, outlet losses, and governing code provisions.
It estimates the cross-sectional area and likely outlet size needed for each downspout based on roof area, rainfall intensity, runoff coefficient, velocity, and the number of downspouts sharing the load.
Pitch increases the actual roof surface relative to plan area. Including a pitch factor helps the tool better represent the collection area feeding the gutter and downspout system.
Use the design storm rainfall intensity required by your project, municipality, or plumbing standard. If multiple values exist, use the governing requirement for the drainage system being designed.
Use a higher value for hard, smooth roofing and a lower value only when surface retention or drainage delay is expected. Many roof drainage designs use values close to 0.90 to 1.00.
Neither is always better. Rectangular profiles fit many conventional gutter systems, while round outlets may suit architectural or specialty drainage layouts. The best choice depends on capacity, detailing, and appearance.
No. It is a design aid. Final sizing should be checked against local code, overflow provisions, gutter capacity, conductor spacing rules, connection details, and any manufacturer limitations.
A safety factor adds conservatism for uncertainty in rainfall, partial blockage, construction tolerance, or future loading conditions. It can help prevent undersizing during severe drainage events.
Size each drainage section according to how water is actually collected and distributed. Separate roof zones, valleys, offsets, or gutter splits should be reviewed individually, then coordinated into the full system.
Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.