Gutter Size Calculator
Calculate the correct gutter width and downspout size for your roof based on drainage area, roof pitch, and local rainfall intensity in inches per hour.
Gutter size selection is driven by the catchment area (roof area draining to that gutter), the roof pitch, and your local design rainfall intensity. Undersized gutters overflow during heavy rain, causing fascia rot, foundation water damage, and basement leaks.
Design flow formula: Q (GPM) = Catchment Area (sq ft) × Rainfall Intensity (in/hr) × 0.0104
(0.0104 converts the unit combination to gallons per minute)
Pitch correction factor (per SMACNA): Adjust catchment area by roof pitch multiplier:
| Pitch | Factor |
|---|---|
| Flat to 3:12 | 1.00 |
| 4:12 to 5:12 | 1.05 |
| 6:12 to 8:12 | 1.10 |
| 9:12 to 11:12 | 1.20 |
| 12:12 and steeper | 1.30 |
Effective catchment area = Horizontal footprint × pitch factor
Gutter capacity at 1 in/hr design rainfall (maximum rated capacity):
| Gutter Size | K-Style Capacity | Half-Round Capacity |
|---|---|---|
| 4-inch | 5,520 sq ft | 5,000 sq ft |
| 5-inch | 7,960 sq ft | 6,500 sq ft |
| 6-inch | 11,520 sq ft | 11,750 sq ft |
| 7-inch | — | 16,325 sq ft |
For higher rainfall intensity: Divide gutter capacity by (Local Design Rainfall ÷ 1) Example: At 2 in/hr design storm, halve the above capacities.
Downspout sizing:
- 2×3 inch rectangular: handles up to 600 sq ft of roof
- 3×4 inch rectangular: handles up to 1,200 sq ft
- 4-inch round: handles up to 1,600 sq ft
- 6-inch round: handles up to 5,000 sq ft
Rule of thumb — downspout spacing: One downspout per 30–40 linear feet of gutter (maximum). Shorter runs = better drainage.
Gutter slope requirement: 1/4 inch drop per 10 feet of horizontal run toward the downspout. For a 40-foot run: 40 × (0.25/10) = 1 inch total drop.
Worked example: Home with a 1,800 sq ft footprint. Roof pitch: 7:12 (factor 1.10). Local 100-year storm: 3.0 in/hr.
- Adjusted catchment area: 1,800 × 1.10 = 1,980 sq ft (per gutter run — assume 4 runs of ~495 sq ft each for a hip roof)
- Gutter capacity needed per run at 3 in/hr: 495 × (3/1) = equivalent to 1,485 sq ft at 1 in/hr
- A 5-inch K-style gutter handles 7,960 sq ft at 1 in/hr → easily handles 1,485 sq ft at 3 in/hr
- Recommendation: 5-inch K-style gutters with 3×4 inch downspouts at each corner