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Gutter Calculator: Length, Downspouts and Cost
Plan new rain gutters: how many feet you need, how many downspouts and what size, plus hangers, elbows, end caps and the slope to set, with an installed cost estimate.
How much gutter do you need?
Gutters run along the eaves, the horizontal edges of the roof. A gable roof has two eaves, one along each long side; a hip roof has eaves on all four sides. Add up the length of every eave you want guttered. A 40 × 30 ft house with a gable roof needs 2 × 40 = 80 ft; with a hip roof it needs the full 140 ft perimeter.
How many downspouts?
The common rule is one downspout for every 30 to 40 ft of gutter, and at least one per separate run. Long runs over 40 ft work best with a downspout at each end, sloping the gutter down from the middle. Then check the roof area each downspout serves:
| Downspout | Roof area it can drain |
|---|---|
| 2 × 3 in | up to about 600 sq ft |
| 3 × 4 in | up to about 1,200 sq ft |
| 3 in round | up to about 700 sq ft |
| 4 in round | up to about 1,250 sq ft |
Based on roughly 1 in of rain per hour. In areas with intense storms, use larger downspouts or more of them.
Worked example
A 40 × 30 ft gable house with a 6/12 roof has about 1,342 sq ft of roof (see our roofing calculator) draining into two 40 ft gutters. Three downspouts (80 ÷ 35, rounded up) each handle about 450 sq ft, so standard 2 × 3 in downspouts are enough. With 10 ft eaves that is 33 ft of downspout, nine elbows and 42 hangers. At $10 per foot for seamless aluminum gutters and $8 per foot for downspouts, the job is about $1,060.
5 inch or 6 inch gutters?
- 5 in K-style is standard for most houses.
- 6 in K-style carries about 40% more water. Choose it for steep roofs (over 8/12), large roof planes, heavy-rain regions, or where leaves clog smaller gutters.
Installation tips
- Slope gutters about ¼ in per 10 ft toward the downspout so water does not pool.
- Hang them every 2 ft (every 18 in where snow loads are heavy), and keep the back edge tucked under the drip edge.
- Send water at least 4 to 6 ft from the foundation with extensions or splash blocks.
Frequently asked questions
How many downspouts do I need?
Plan one downspout for every 30 to 40 ft of gutter, and at least one per separate gutter run. Each 2 × 3 in downspout handles about 600 sq ft of roof; a 3 × 4 in handles about 1,200 sq ft.
How much slope should gutters have?
About ¼ in of fall for every 10 ft of gutter toward the downspout. Runs over 40 ft are often pitched from the middle down to a downspout at each end.
How much do gutters cost?
Seamless aluminum gutters typically cost $7 to $15 per foot installed in the US. Copper and steel cost more; vinyl DIY sections cost about $3 to $5 per foot.