Rainfall intensity is the heaviest short-burst rainfall you plan for. In the US you can look it up for your location in NOAA Atlas 14 (the Precipitation Frequency Data Server); local gutter installers also know the usual figure.
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
What is the roof pitch factor?
1.0 for up to 3/12, 1.05 for 4/12–5/12, 1.1 for 6/12–8/12, 1.2 for 9/12–11/12 and 1.3 for 12/12 and steeper.
How much can a 5 inch K-style gutter handle?
About 5,520 adjusted sq ft (roof area × pitch factor × rainfall intensity). A 6-inch K-style handles about 7,960.
What if my roof is too big for 6 inch gutters?
Split the roof into smaller drainage sections, add a gutter break in the middle with downspouts at both ends, or use 7-inch commercial gutters.
Is GutterCalc free?
Yes, with no sign-up or ads.
Are my measurements saved?
Yes, in this browser only.