| 5" gutter | Big enough (up to 834 sq ft) |
|---|---|
| 6" gutter | Big enough (up to 1,280 sq ft) |
| Gutter slope used | 1/16" per foot |
| Roof area per downspout | 600 sq ft with 1 downspout |
| 2 × 3 in rectangular downspout | Big enough (up to 1,840 sq ft each) |
| 3 in round downspout | Big enough (up to 2,930 sq ft each) |
| 3 × 4 in rectangular downspout | Big enough (up to 4,400 sq ft each) |
| 4 in round downspout | Big enough (up to 6,130 sq ft each) |
1/16" per foot is the flattest slope in the code table. If your gutter is hung flatter than that, the table doesn't cover it, so treat this size as the minimum and lean toward the bigger size when it's close.
The code table rates half-round gutters. For K-style gutters, check the maker's capacity rating. Your local code may also use a different edition, so check with your building department.