Gutter Downspout & Drainage Calculator
Calculate the number of downspouts and drain capacity needed based on your roof area and local rainfall intensity.
Drainage Requirements
Gutter and downspout sizing depends on your roof area and how much rain falls in your region.
Water Volume (gal/hr) = Roof Area (sq ft) x Rainfall (in/hr) x 0.623
Downspouts Needed = Water Volume / Downspout Capacity
Downspout capacities (at standard slope):
- 2x3 inch rectangular: 600 gal/hr
- 3x4 inch rectangular: 1,200 gal/hr
- 3 inch round: 700 gal/hr
- 4 inch round: 1,200 gal/hr
Rainfall intensity guidelines:
- Light rain areas: 1 in/hr (2.5 cm/hr)
- Moderate: 2-4 in/hr (5-10 cm/hr)
- Heavy (tropical/storm): 4-8 in/hr (10-20 cm/hr)
General rule: One downspout per 20-30 ft of gutter run, or per 600-800 sq ft of roof area.