Gutter Calculator

How the gutter calculator works

A typical house with 120 linear feet of gutter needs 4 downspouts, 60 hidden hangers, 2 outside corners, and 2 end caps. Downspouts go in at one per 35 feet of run, hangers at 24 inches on center, and 5-inch K-style aluminum handles the drainage for most residential roofs. Add one 3-ft downspout extension per downspout to carry water away from the foundation.

Gutter runs linear feet along your roof eaves. Add an outside corner for each external (90° outward) turn, an inside corner for each internal (90° inward) turn. End caps cap the open ends of any gutter run that doesn't dump into a downspout or another section.

Gutter profile and size

Gutter capacity is a cross-sectional area problem. The trough has to hold the peak flow off the roof plane above it long enough for the downspouts to drain it, and the profile determines how much area you get for a given nominal size. This is why a 6" half-round and a 6" K-style are not interchangeable despite the shared number — the K-style's flat back and squared bottom hold considerably more water than a semicircle of the same width.

The half-round figures are geometry, not marketing: a 6-inch half-round is a semicircle of 3-inch radius, so its area is π × 3² ÷ 2 = 14.1 square inches — roughly 40% less than a 6-inch K-style. If you are switching from K-style to half-round for looks, either go up a size or add downspouts, because you are removing capacity from the system.

Two factors push you up a size regardless of roof area. A steep roof delivers water faster and with more horizontal momentum, so water overshoots a shallow trough — 8:12 and above is a size-up. A valley that dumps into a short section of gutter concentrates the drainage of two roof planes into a few feet of trough, which is why the gutter under a valley overflows in a storm while the rest of the house looks fine. A diverter at the valley or an oversized section is the fix.

Downspout sizing

Rule of thumb: 1 downspout per 35–40 linear feet of gutter (1 per 10–12 m). For roofs over 1,500 sq ft (140 m²) of drainage area, bump to 1 per 25 ft. Heavy rain regions (Pacific Northwest, Gulf Coast) bump to 1 per 25 ft regardless.

The spacing rule is a proxy for the real one, which is based on area. The trade standard is roughly 1 square inch of downspout cross-section per 100 square feet of roof drainage area in ordinary rainfall, tightening toward 1 square inch per 70–80 sq ft where storms are intense. Work the downspout count from area and use the spacing rule to check yourself:

Note that a 3×4 downspout carries twice the water of a 2×3 — the standard box-store size — for perhaps a 30% price increase, and it clogs far less readily because a wet oak leaf will pass through it. If you are replacing gutters and have historically had overflow problems, upsizing the downspouts is usually a better spend than upsizing the trough.

Roof drainage area is not roof area. For a sloped roof, the area that matters for drainage is the horizontal footprint the roof covers, not the surface measured up the slope — rain falls vertically. On a steep roof the difference is substantial. Our roof area calculator gives you both figures; use the footprint for gutter sizing and the sloped area for shingles.

Slope, and why it decides whether the system works

A gutter is not level. It has to fall toward its downspout, and the accepted residential spec is about ¼ inch of drop per 10 feet of run — roughly 1/16" per foot. Over a 40-foot run that is a full inch of drop, which is visible if you look for it and invisible if you do not, because the fascia line reads as the reference and the eye forgives a small taper.

• Too little slope and water stands in the trough. Standing water breeds mosquitoes, holds debris in place until it composts into a plug, and adds dead weight that pulls hangers out of the fascia.
• Too much slope and the gutter looks crooked against the roof line, and on long runs the far end sits so low it no longer catches the water coming off the roof above it.
• Long runs get a high point in the middle. Past about 40 feet, put a downspout at each end and crown the run in the centre so it drains both ways, rather than dragging one continuous fall across the whole span.

Snap a chalk line before you hang anything. Mark the high end at your intended gutter height, calculate the total drop for the run, mark the low end, and snap between them. Hanging by eye off the fascia is the single most common reason a new gutter puddles.

Hangers and brackets

Modern gutters use hidden hangers — internal brackets screwed up into the fascia through the inside back of the gutter. Space at 24" (60 cm) OC; closer in heavy snow regions or on long unsupported runs.

Tighten the spacing to 18 inches where snow and ice load the trough, where the gutter runs under a metal roof that sheds in slabs, and on any run over about 30 feet. The screw matters as much as the spacing: a hidden hanger wants a 2½-inch screw driven into solid fascia, and ideally into the rafter tail behind it. Fascia that is soft, spliced, or already carrying old spike holes will not hold, and re-hanging new gutter on rotten fascia is the most common way a gutter job fails within two years. Probe the fascia with an awl every few feet before you order material — if it goes in easily, budget for fascia replacement in the same project.

Worked example: a 180-ft run in a wet climate

A larger two-storey house with 180 linear feet of gutter line, four outside corners, two inside corners, and two open ends, in a region where you tighten downspouts to one per 25 feet. Set total gutter run 180, outside corners 4, inside corners 2, end caps 2, and change downspout spacing to 25 ft in Advanced options.

Downspouts = 180 ÷ 25 = 7.2, rounded up to 8 downspouts, with 8 three-foot extensions to match. Hangers at the default 2-ft spacing = 180 ÷ 2 = 90 hidden hangers. Add the 4 outside and 2 inside corners and 2 end caps. Sanity-check against the area rule: if this house has a 2,400 sq ft roof footprint, eight 3×4 downspouts give 96 square inches of drainage for 2,400 sq ft — comfortably better than the 1-per-100 baseline, which is the right side to err on in a wet climate.

Worked example: a detached garage in snow country

A detached gable-roofed garage with 22-ft eaves guttered on both sides gives 44 linear feet of run in two independent straight sections — no corners at all, but four end caps, one at each open end. Leave downspout spacing at 35 ft but tighten the hanger spacing to 1.5 ft for snow load. Set gutter run 44, outside corners 0, inside corners 0, end caps 4.

Downspouts = 44 ÷ 35 = 1.26, rounded up to 2 downspouts — which is exactly right here, one per side, so each 22-ft run drains on its own. Hangers = 44 ÷ 1.5 = 29.3, rounded up to 30 hangers, against the 22 you would have used at 24-inch spacing. That extra eight hangers costs a few dollars and is the difference between a gutter that survives a winter of sliding snow and one that peels off the fascia in February. Add 2 three-foot extensions so the discharge lands clear of the slab edge.

Where the water goes after the downspout

Gutters solve a roof problem by creating a concentrated ground problem, and a downspout that dumps at the foundation is worse than no gutter at all, because it puts the entire roof's water into one spot against the wall. The calculator gives you one 3-ft extension per downspout as a minimum; that is the floor, not the target.

• Extensions — cheap and effective, but only if they run past the backfill zone. Four to six feet is a realistic minimum on most lots.
• Splash blocks — stop erosion at the discharge point but do nothing about where the water goes next. Use them with an extension, not instead of one.
• Buried solid drain line — the durable answer where grade allows: 4" solid (not perforated) pipe from the downspout to daylight or a dry well, sloped at least 1% and with a cleanout.
• Grade before plumbing. The ground should fall roughly 6 inches over the first 10 feet from the foundation. If it does not, no drainage detail will save you.

Failure modes and when to call a pro

Most gutter problems are diagnosable from the ground during a storm. Water sheeting over the front edge in a downpour is undersized gutter or overshoot from a steep roof; water dripping steadily behind the gutter is a missing drip edge or a rotted fascia; a section that stays wet and green days after rain is a slope problem; a downspout that gurgles and backs up is a clog or an undersized outlet.

Hire this out if the run is on a second storey or above, if you would be working off a ladder on a steep or unstable surface, if the fascia or roof sheathing edge turns out to be rotten (that is a roofing repair, not a gutter job), or if you want seamless gutter, which requires an on-site roll-forming machine that is not practical to rent for one house. Falls from ladders are the real hazard in this trade — more so than anything about the gutter itself. If you are not completely confident on a ladder at that height, the few hundred dollars an installer charges is not the expensive option.

Common questions

For the roof above the gutters, see the roof area calculator or shingles calculator.