Gutter Size Calculator
Determine the recommended gutter width based on your roof area and local rainfall intensity.
Covers 5-inch and 6-inch gutters.
How Gutter Sizing Works
Gutters must handle the peak rainfall rate from your roof without overflowing. Choosing the wrong size leads to water damage at the foundation and fascia rot. The key variable is roof drainage area combined with your local rainfall intensity.
Effective drainage area formula:
Effective Area (sq ft) = Roof Length × Width × Pitch Factor
Roof pitch factors:
| Pitch (rise:run) | Factor |
|---|---|
| Flat to 3:12 | 1.0 |
| 4:12 to 5:12 | 1.05 |
| 6:12 to 8:12 | 1.1 |
| 9:12 to 11:12 | 1.2 |
| 12:12 and steeper | 1.3 |
Worked example:
- Roof section: 40 ft long × 20 ft wide (measured horizontally)
- Roof pitch: 6:12
- Effective area = 40 × 20 × 1.1 = 880 sq ft
Gutter capacity guidelines (U.S. standard):
| Gutter Width | Max Drainage Area |
|---|---|
| 4 inch | up to 480 sq ft |
| 5 inch | up to 680 sq ft |
| 6 inch | up to 960 sq ft |
For the 880 sq ft example, a 6-inch gutter is the correct size.
Downspout sizing:
- 1 downspout per 20–30 ft of gutter run (standard)
- Each 3×4 inch rectangular downspout handles ~600 sq ft of roof area
- Each round 4-inch downspout handles ~400 sq ft
High-rainfall regions (Pacific Northwest, Florida, Gulf Coast) should add one size larger than the formula suggests — design for the 100-year storm event, not average rainfall.
Gutters with debris guards can run full-flow even when partially clogged, which effectively reduces their rated capacity by 20–30%.