Gutter and Downspout Calculator

How to use the Gutter Calculator

Start with Length: add up every horizontal eave that gets a gutter. Rakes do not, water runs down them, not along them. The downspout footage comes from the eave height plus a few feet of elbow and extension at the bottom.

Then use Sizing. This is the part most gutter estimates skip. Capacity depends on the plan area of roof feeding the gutter, how steep that roof is, and how hard it rains where you live. None of those appear in a length measurement.

Slope checks the other constraint: a gutter must fall toward the outlet, but the fall over a long run can drop the far end out of sight behind the fascia.

Gutter Cost prices gutter, downspout and hangers as three separate lines. Hanger spacing is the one to watch: moving from 36 in to 24 in centres is half as many again, and it is the right call anywhere that carries snow.

Gutter formula

• gutter = total eave length
• hangers = ceiling(run ÷ spacing) + 1 per section
• downspout footage = (eave height + 3 ft) × downspouts
• adjusted area = roof plan area × pitch factor
• capacity = base capacity ÷ rainfall intensity

A steeper roof throws water at the gutter faster, so the same plan area behaves like a larger one.

Worked example

A 1,600 sq ft roof at 7 : 12 in a region that sees 5 in/hr:

• Adjusted area: 1,600 × 1.10 = 1,760 sq ft
• 5 in K-style capacity: 5,520 ÷ 5 = 1,104 sq ft, not enough on one run
• 6 in K-style capacity: 7,960 ÷ 5 = 1,592 sq ft
• 2 × 3 downspout: 3,000 ÷ 5 = 600 sq ft each → 3 downspouts
• 3 × 4 downspout: 6,000 ÷ 5 = 1,200 sq ft each → 2 downspouts

Even 6 in is 168 sq ft short on a single run. Split the roof between two runs, which is how most houses are drained anyway, and each side carries 880 sq ft, well inside a 5 in gutter. That split is exactly what the sizing panel is for.

What the result means

Capacity is per run, not per house. A gutter that drains half the roof only has to carry half the water. Splitting a long eave with two downspouts at opposite ends halves the load on each side.

Downspouts are usually the bottleneck. A 2 × 3 downspout has 6 sq in of throat; a 3 × 4 has 12. Going up one size on the downspout does more for overflow than going up on the gutter.

Rainfall intensity varies enormously. The Pacific Northwest is famously wet but its peak intensity is modest. The Gulf Coast can see nine inches an hour in a five-minute burst, and that burst is what overflows a gutter.

Common mistakes

• Measuring the rake. Gutter goes on horizontal eaves only.
• Sizing on the whole roof when the roof drains to four different runs. That over-buys. The opposite error, sizing one long run on a quarter of the area, overflows.
• Using the sloped roof area. Rain falls vertically, so the plan area is what catches it. The pitch enters as a factor, not as the sloped area.
• Too much fall. Half an inch per ten feet is plenty; SMACNA’s minimum is a sixteenth of an inch per foot, which is five eighths per ten. An inch per ten feet over a 40 ft run drops the outlet end four inches, which shows.
• Hangers too far apart. Two feet is standard, closer where snow and ice load the gutter. A full gutter of water weighs about 2.8 lb per foot at 5 in.

Frequently asked questions