How many downspouts do I need?

Plan on one per about 30–40 feet of gutter — roughly one per 600–800 sq ft of roof drained. Long runs and big roofs need more so water doesn't overrun the gutter in a heavy rain.

How far apart should gutter hangers go?

About every 24 inches, with a hanger at each end of every run (so it's one more than the number of gaps). Tighten to 16–18 inches in regions with snow and ice — a loaded gutter is what pulls away from the fascia.

Should I get 5-inch or 6-inch gutter?

5-inch K-style handles most homes; step up to 6-inch (with 3×4-inch downspouts) for large or steep roofs that shed a lot of water fast. Capacity is gated by the downspout as much as the trough, so size them together.

Gutter Calculator

Add up your roof's eaves and this works out the gutter to buy, plus the downspouts and hangers to hold it.

Gutter (10-ft sections)

12sections

Downspouts: 4 downspouts
Hangers: 61 hangers
Total run: 120 linear ft

How these numbers are worked out

The formula

Gutter sections = ⌈ total run ÷ 10 ⌉ — sectional K-style gutter is sold in 10-foot lengths (seamless gutter is roll-formed to the run, so go by the total linear feet). Downspouts = ⌈ run ÷ spacing ⌉, one per ~30–40 ft. Hangers = ⌈ run ÷ spacing ⌉ + 1 — a bracket at each end of the run — spaced about every 24 inches, tighter (16–18 in) in snow country.

Constants and sources

Sectional gutter sold in 10-ft lengths (seamless = total linear ft). Conventions: a downspout per ~30–40 ft of run (~600–800 sq ft of roof); a hanger every ~24 in plus one for both ends, tighter in snow. Verify against local rainfall/code.

How it was checked

Last reviewed June 16, 2026. Before this calculator was published, its formula, constants and worked example were checked in a separate, AI-assisted review that redid the arithmetic and tested the factual claims; the site owner then approved it for publishing. No engineer or tradesperson reviews these pages. Where a figure depends on the product (a density, a coverage rate, a bag size), it is either an input you can change above or a fixed planning value stated on this page, so check it against your product's label or your supplier.

Spot a wrong number or an outdated figure? Send a correction.

Worked example

120 ft of eaves: ⌈120 ÷ 10⌉ = 12 sections; ⌈120 ÷ 35⌉ = 4 downspouts; at 24-in hanger spacing, ⌈(120 × 12) ÷ 24⌉ + 1 = 60 + 1 = 61 hangers.

FAQs

How many downspouts do I need?: Plan on one per about 30–40 feet of gutter — roughly one per 600–800 sq ft of roof drained. Long runs and big roofs need more so water doesn't overrun the gutter in a heavy rain.
How far apart should gutter hangers go?: About every 24 inches, with a hanger at each end of every run (so it's one more than the number of gaps). Tighten to 16–18 inches in regions with snow and ice — a loaded gutter is what pulls away from the fascia.
Should I get 5-inch or 6-inch gutter?: 5-inch K-style handles most homes; step up to 6-inch (with 3×4-inch downspouts) for large or steep roofs that shed a lot of water fast. Capacity is gated by the downspout as much as the trough, so size them together.