Gutter is measured along the eaves — the roof edges that shed water — and the other half of the job is spacing enough downspouts to carry that water away before the gutter overflows. Here is how to work out the run, the downspouts and what it costs.
How much gutter
Add up the eaves that get a gutter. On a simple gable roof that is the two long sides — 2 × the building length; on a hip roof it is all four sides, 2 × (length + width). A 40 × 30 ft house needs 80 ft of gutter on the two long sides, or 140 ft all the way round.
How many downspouts
One downspout per 30–40 ft of gutter, and at least one at each end of a run — so water drains before it backs up in a downpour. That 80 ft of gutter wants two or three. Bigger roofs and heavy-rain areas want them closer together.
Gutter & Downspout Cost Calculator
Enter your building size for the gutter run, downspouts, sections and the cost per foot.
Sold by the foot or in sections
Sectional gutter comes in 10 ft lengths (also 5 ft) that you join, so divide the run by the section length and round up. Seamless gutter is rolled to length on site from a machine — no joints, priced by the foot. Either way, add corners, end caps, hangers and elbows to the order.
What it costs
Installed, aluminum K-style gutter runs roughly $6–$12 per foot, seamless a little more, copper far more; downspouts are similar per foot. Materials alone are about $2–$5 per foot for aluminum. Enter a price per foot for the gutter and a price per downspout for a material-plus-labor total.
Pick the profile for the roof
The run and downspout count are only half of it — the gutter profile has to carry the water. Most homes use 5 in K-style with 2×3 or 3×4 in downspouts; a big or steep roof that sheds a lot of water fast steps up to 6 in gutter and 3×4 in downspouts. Slope the gutter about ¼ in per 10 ft toward the downspouts — a dead-flat gutter pools and overflows.