Reference
Gutter and Downspout Sizing Table
Gutter and downspout capacity by rainfall intensity, with the pitch allowance, so a gutter can be sized for the roof face it serves.
The method
Use the plan (horizontal) area of the roof face that drains into the gutter, not the sloped area. Rainfall intensity is the 5-minute, 10-year figure for your location from NOAA Atlas 14, in inches per hour.
Pitch allowance
| Roof pitch | Multiply plan area by |
|---|---|
| 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 |
Gutter capacity in square feet of design area
| Rainfall intensity | 5 in K-style | 6 in K-style | 5 in half-round | 6 in half-round |
|---|---|---|---|---|
| 1 in/hr | 5,520 | 7,960 | 2,500 | 3,840 |
| 2 in/hr | 2,760 | 3,980 | 1,250 | 1,920 |
| 3 in/hr | 1,840 | 2,653 | 833 | 1,280 |
| 4 in/hr | 1,380 | 1,990 | 625 | 960 |
| 5 in/hr | 1,104 | 1,592 | 500 | 768 |
| 6 in/hr | 920 | 1,327 | 417 | 640 |
| 7 in/hr | 789 | 1,137 | 357 | 549 |
| 8 in/hr | 690 | 995 | 313 | 480 |
| 10 in/hr | 552 | 796 | 250 | 384 |
Capacities are for level gutters; a slight slope adds a little margin.
Downspouts
| Downspout | Cross-section | Roof served (trade rule, about 100 sq ft per sq in) |
|---|---|---|
| 2 × 3 in rectangular | 6 sq in | about 600 sq ft |
| 3 × 4 in rectangular | 12 sq in | about 1,200 sq ft |
| 3 in round | 7.1 sq in | about 700 sq ft |
| 4 in round | 12.6 sq in | about 1,250 sq ft |
The 100 sq ft per square inch rule is the gutter trade’s rule of thumb for a typical design storm; reduce it where storms are intense. Whatever the capacity, no run should go more than about 30 to 40 feet per downspout.
Runoff volume
One inch of rain on one square foot of plan roof area is 0.623 US gallons (1 mm on 1 m² is 1 litre).
Sources
Gutter capacities as published from the SMACNA Architectural Sheet Metal Manual method by gutter manufacturers; SMACNA roof pitch factors; NOAA Atlas 14 precipitation frequency estimates for intensity; the downspout figure is the industry rule of thumb. Plumbing codes size roof drains with a 60-minute, 100-year rainfall instead; do not mix the two design storms.
Calculators that use this table
- The downspout count and size calculator checks gutter capacity and counts downspouts.
- The gutter length calculator counts the parts once the size is chosen.
- The rain barrel calculator uses the runoff volume.
- The gutter slope calculator sets the fall to each downspout.
The guide gutter sizing by roof area walks through the method.