Each downspout usually needs three elbows: two at the top to bring it back to the wall and one at the bottom to send water away from the foundation.
How the gutter calculator works
GutterCalc multiplies the drainage area (gutter length × roof depth) by the pitch factor and the rainfall intensity, then picks the smallest gutter of your style whose capacity is at least that adjusted area. Downspouts = the larger of the gutter length ÷ the maximum run per downspout and the roof area ÷ the downspout capacity, rounded up. Hangers = length ÷ spacing + 1. Slope = ¼ inch (or your figure) per 10 feet over the run to each downspout.
Gutter capacity (adjusted sq ft)
| Gutter | Capacity |
|---|---|
| 5″ K-style | 5,520 |
| 6″ K-style | 7,960 |
| 5″ half-round | 2,500 |
| 6″ half-round | 3,840 |
Pitch factor
| Roof pitch | Factor |
|---|---|
| Flat to 3/12 | 1 |
| 4/12 – 5/12 | 1.05 |
| 6/12 – 8/12 | 1.1 |
| 9/12 – 11/12 | 1.2 |
| 12/12 and steeper | 1.3 |
Downspout capacity (rule of thumb)
| Downspout | Opening | Roof area |
|---|---|---|
| 2 × 3″ rectangular | 6 sq in | ≈ 600 sq ft |
| 3 × 4″ rectangular | 12 sq in | ≈ 1,200 sq ft |
| 3″ round | 7.1 sq in | ≈ 700 sq ft |
| 4″ round | 12.6 sq in | ≈ 1,250 sq ft |
Capacities, pitch factors and the 1 sq in per 100 sq ft downspout rule are widely published rules of thumb for residential gutters, not a code calculation; local codes (for example the plumbing code's storm drainage tables) and your installer may size differently. Results are estimates.
Frequently asked questions
How many downspouts for a 60 ft gutter?
At least two: one per 40 ft of gutter at most, and two 2 × 3 downspouts for 1,200 sq ft of roof. With 3 × 4 downspouts one could carry the water, but two shorten the run.
Should I use 2 × 3 or 3 × 4 downspouts?
3 × 4 carries about twice as much and clogs less, so it is a good choice with 6-inch gutters or under trees.
How far should downspouts discharge from the house?
Commonly at least 4 to 6 feet from the foundation, or into a drain. Check local rules.
Is GutterCalc free?
Yes, with no sign-up or ads.
Are my measurements saved?
Yes, in this browser only.