Fix hangers into the rafter tails where you can, not only into the fascia board, and keep one within a few inches of each end and each seam.
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 gutter hangers for 50 feet?
At 24-inch spacing, 600 ÷ 24 = 25, plus one for the end: 26 hangers.
What spacing for gutter hangers in snow areas?
About 18 inches, or follow the hanger maker's guide for snow loads.
Do seamless gutters need fewer hangers?
No. Seamless gutters still need the same spacing; they just have no seams to leak.
Is GutterCalc free?
Yes, with no sign-up or ads.
Are my measurements saved?
Yes, in this browser only.