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Gutter Calculator
Quick answer: Measure the total length of your roof eaves for gutter footage, add one downspout per 30–40 ft of gutter and a hanger every 24–36 in. Round up to 10-ft gutter sections.
Estimate linear feet of gutter, the number of downspouts, hangers, elbows, and end caps for your roof — so you buy the right materials in one trip.
Results update automatically as you type.
How gutter estimating works
Four numbers cover almost every gutter job: the linear feet of gutter (your total eave length), the number of downspouts (driven by both gutter length and roof area), the hangers that hold it up, and the fittings (end caps and elbows). Get those right and the material list falls into place.
Sizing your downspouts
Downspouts are the part most people under-buy. Use the stricter of two rules: one downspout per 30–40 feet of gutter, and one per 600–800 sq ft of roof draining into that gutter. A big roof on a short eave still needs enough downspouts to move the water.
Hangers, end caps, and elbows
- Hangers: every 24 inches (every 18 inches in snow country) — roughly one per two linear feet.
- End caps: two per separate gutter run (left and right).
- Elbows: about three per downspout to get from the gutter, over the eave, and down the wall.
5-inch vs 6-inch gutters
Five-inch K-style handles most homes. Step up to six-inch for large roof areas, steep pitches that dump water fast, or high-rainfall regions — the extra capacity prevents overflow during heavy storms.
Drainage area, not roof area
Gutter sizing is driven by how much water arrives at a given run, and that depends on the roof area draining into that run — not the total roof. A 2,000 sq ft roof split evenly across four runs presents each gutter with 500 sq ft. Get the split wrong and you undersize the one run that takes half the house.
Two adjustments matter. Steeper roofs deliver water faster, so the effective drainage area is multiplied by a pitch factor (roughly 1.1 at 6/12, 1.2 at 9/12, 1.3 at 12/12). And local rainfall intensity — the peak hourly rate, not the annual total — sets the baseline. A Gulf Coast house and a Pacific Northwest house can have similar annual rainfall while needing very different gutters, because one gets it in violent bursts and the other gets drizzle.
When 5-inch is not enough
| Gutter | Cross-section | Typical capacity | Use when |
|---|---|---|---|
| 5 in K-style | ~ 6.8 sq in | Up to ~750 sq ft per downspout | Most single-storey homes, moderate rainfall |
| 6 in K-style | ~ 9.8 sq in | Up to ~1,400 sq ft per downspout | Steep or large roofs, heavy rainfall regions, metal roofs |
| 5 in half-round | ~ 9.6 sq in | Lower than K-style of same width | Historic and architectural applications |
A 6 in gutter carries roughly 40% more water than a 5 in for a modest price increase, and pairs with 3×4 in downspouts that clog far less readily than 2×3 in. On any metal roof, size up by default — water sheets off standing seam much faster than it does off shingles.
Slope, seams, and the details that cause failures
- Slope: about 1/16 in per foot toward the downspout — roughly ½ in over a 10 ft run. Too flat and water stands and breeds; too steep and it looks wrong against the fascia.
- Hanger spacing: 24–36 in on centre normally, tightened to 18–24 in in snow regions. Sagging gutters are almost always a hanger-spacing failure, not a gutter failure.
- Downspout placement: one per 30–40 ft of run maximum, and at both ends of any long run rather than one in the middle.
- Discharge: extend at least 4–6 ft from the foundation. A correctly sized gutter that dumps at the footing has simply relocated the problem.
Frequently asked questions
How many downspouts do I need?
At least one per 30–40 ft of gutter and one per 600–800 sq ft of roof. Long or steep roofs need more.
How far apart should hangers be?
Every 24 inches normally; every 18 inches in heavy-snow areas.
Do I need 5-inch or 6-inch gutters?
5-inch suits most homes; 6-inch for large or steep roofs and heavy rainfall.