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Gutter Size Calculator Guide
Gutter and downspout sizing from the roof area that actually catches rain — which is not the same area you'd use to buy shingles.
Rain falls vertically. A steep roof does not catch more rain — it catches the same rain and delivers it faster.
For material you use the sloped surface area (× the pitch multiplier).
For drainage you use the horizontal projection — the footprint.
A 40×30 roof drains 1,200 sq ft even though it has 1,342 sq ft of shingles on it.
Footprint, Not Surface Area
This trips up people who've just done a roofing calculation. You measured 1,342 sq ft of roof to buy shingles, so it feels natural to size gutters off the same figure.
But a raindrop falling straight down doesn't care about the angle it lands on. The roof intercepts a column of falling water whose cross-section is the footprint. Pitch matters for gutters only because a steep roof throws water further and faster — a wind-driven-rain effect, not a geometry one.
Rainfall Intensity Is Local and It Dominates
This is the input that actually decides the answer, and it varies enormously — a roof in a dry temperate region and an identical roof in a subtropical storm belt need genuinely different gutters.
What you want is your local peak short-duration intensity, usually quoted as the 5-minute, 100-year figure in inches per hour. Your local building department or meteorological service publishes it. A default here is only a placeholder — this is the number worth looking up properly.
Downspouts Are the Bottleneck, Not the Gutter
An oversized gutter feeding one undersized downspout overflows, every time. The gutter is a holding trough; the downspout is the drain.
A widely used rule is about one square inch of downspout cross-section per 100 sq ft of roof. A 2×3 downspout is 6 sq in and covers roughly 600 sq ft; a 3×4 is 12 sq in and covers about 1,200. Two small downspouts genuinely beat one large gutter.
Where They Go Matters As Much As How Many
Downspouts at both ends of a long run halve the distance water travels and halve the depth it builds. One in the middle of a run does the same job.
Gutters also need a slope — a gentle fall toward each outlet, on the order of a quarter inch per ten feet. A dead-level gutter holds standing water, which breeds mosquitoes, grows things, and adds weight that eventually pulls the fascia away.
Where the Water Goes Afterwards
Perfect gutters dumping against the foundation just deliver the problem more efficiently. Extensions, splash blocks or a drain line matter as much as the sizing.
Related: roof shingle calculator, metal roofing calculator, french drain calculator.
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