Gutter Size Calculator
Calculate the right gutter size for your roof based on area, pitch, and local rainfall intensity.
Recommended Gutter Size
Gutter sizing depends on your roof’s drainage area and local rainfall intensity.
Adjusted Roof Area accounts for roof pitch:
Adjusted Area = Roof Length × Roof Width × Pitch Factor
Pitch factors:
| Roof Pitch | Factor |
|---|---|
| Flat–3/12 | 1.0 |
| 4/12–5/12 | 1.05 |
| 6/12–8/12 | 1.1 |
| 9/12–11/12 | 1.2 |
| 12/12+ | 1.3 |
Gutter capacity (at standard slope):
| Gutter Size | Max Drainage Area (sq ft) |
|---|---|
| 5-inch K | Up to 5,520 sq ft |
| 6-inch K | Up to 7,960 sq ft |
| 5-inch half-round | Up to 2,500 sq ft |
| 6-inch half-round | Up to 3,840 sq ft |
Downspout rule: One downspout per 20–30 feet (6–9 m) of gutter run. Each downspout handles about 1,200 sq ft of roof area.