Gutter Calculator
Two questions in one: what size gutter your roof actually needs, and how much of everything to buy. Enter your gutter run, the roof area draining into it, the pitch and your local rainfall intensity.
Results
How gutter sizing works
Most people order 5-inch K-style gutter because that is what the store stocks, and on a big or steep roof in a wet region it overflows every heavy storm. Gutter size is not a style choice — it is a drainage calculation, and it depends on three things: how much roof drains into the gutter, how steep that roof is, and how hard it rains where you live.
Formula: plan roof area × pitch factor = adjusted roof area. Compare that against the profile's capacity, which is capacity constant ÷ rainfall intensity. A 5" K-style gutter carries about 5,520 divided by the local intensity — so 1,380 sq ft at 4 in/hr, but only 920 sq ft at 6 in/hr.
The pitch factor is there because a steep roof throws water at the gutter faster than a flat one, so the same footprint behaves like a larger roof.
| Profile | At 3 in/hr | At 4 in/hr | At 6 in/hr |
|---|---|---|---|
| 4" K-style | 960 | 720 | 480 |
| 5" K-style | 1,840 | 1,380 | 920 |
| 6" K-style | 2,653 | 1,990 | 1,327 |
| 5" half-round | 833 | 625 | 417 |
| 6" half-round | 1,280 | 960 | 640 |
Downspouts, hangers and the rest of the order
- Downspouts are the other half of the problem. A 2×3 in downspout drains about 600 sq ft of roof; a 3×4 in drains about 1,200 sq ft. Put one at least every 35 ft of run regardless of area — a long gutter with a single outlet backs up in the middle.
- Hangers go every 24 inches where snow and ice load the gutter, and up to 36 inches in mild climates. Cheap gutter jobs are usually under-hung, and that is what pulls the gutter away from the fascia.
- Gutter comes in 10 ft sections (or seamless, rolled on site). Add two end caps per run, plus inside and outside corner miters where the gutter turns.
- Slope the run about ¼ inch per 10 feet toward the downspout — enough to drain, not enough to see from the ground.
- Sizing the roof above it? Use the roofing calculator for the true sloped area, or the metal roof calculator if you are going metal.
Frequently asked questions
What size gutter do I need — 5 inch or 6 inch?
It depends on the adjusted roof area draining into it. A 5-inch K-style gutter handles about 1,380 sq ft at a 4 in/hr rainfall intensity, a 6-inch about 1,990 sq ft. Large roofs, steep roofs and high-rainfall regions push you to 6 inch — and if you are near the limit, oversize.
How many downspouts do I need?
One 2×3 in downspout per roughly 600 sq ft of roof, or one 3×4 in per 1,200 sq ft — and at least one for every 35 feet of gutter run, whichever gives the higher number. A long run with a single outlet will back up in the middle during a real storm.
How far apart should gutter hangers be?
Every 24 inches in regions with snow and ice, up to 36 inches in mild climates. Ice and standing water are heavy, and under-hung gutter is the usual reason a gutter sags or pulls off the fascia.
What rainfall intensity should I use?
Use the 5-minute, 100-year rainfall figure for your area. Roughly: 3 in/hr in the Pacific Northwest and Northeast, 4 in/hr across most of the country, 5–6 in/hr in the Midwest and Southeast, and 7 in/hr on the Gulf Coast and in South Florida.