Gutter Calculator

How gutter sizing works

Two separate limits decide how many downspouts a roof needs, and both have to be satisfied. The first is distance: water has to travel along the trough to reach an outlet, so you want one downspout per 35 feet of run. The second is volume: each downspout can only pass so much water, and that is set by the size of its opening.

The opening rule is simple arithmetic. A downspout drains about 100 square feet of roof per square inch of cross-section. A 2 by 3 inch downspout is 6 square inches, so 600 square feet. A 3 by 4 is 12 square inches, so 1,200. That is why the two figures are exactly double each other, and it is why a 3 by 4 is worth far more than its size difference suggests.

A steeper roof does not collect more rain

This is the part that gets stated backwards almost everywhere, so it is worth being precise. Rain falls vertically. A roof collects water over its horizontal footprint, not over its sloped surface, so a 12/12 roof and a flat roof on the same 40 by 30 footprint catch exactly the same rain. Sizing a gutter off the sloped surface area oversizes the whole system.

What pitch does change is how the water behaves once it lands: it arrives at the eave faster and with more force, and wind drives more rain onto a steep surface than a shallow one. Gutter sizing handles that with a pitch factor applied to the footprint, and it is a modest adjustment.

The right-hand column is the pitch multiplier our roofing calculator uses, shown so the two are not confused. Our roof pitch chart lists that multiplier for every pitch from 1/12 to 12/12, along with the angle in degrees. That one converts a footprint into actual roof surface, which is what you buy shingles by. It is the wrong number for gutters, and at 12/12 it is 1.414 against the 1.30 a gutter wants.

Picking 5 inch or 6 inch

Five inch K-style with 2 by 3 downspouts is the default on most houses and it is genuinely adequate. This calculator steps up to 6 inch with 3 by 4 downspouts when one of three things is true, and it tells you which one fired:

The water needs more downspouts than the run spacing lets you place. If a 2 by 3 downspout every 35 feet still leaves more than 600 square feet of drainage on each one, the outlets are the bottleneck and a bigger trough with bigger outlets is the fix. The roof is 9/12 or steeper. Water comes off a steep roof fast enough to shoot past the front lip of a narrow trough, which is why the same house can take 5 inch at 8/12 and want 6 inch at 9/12. Or a single run is longer than 40 feet, where flow accumulates before it reaches an outlet.

Whatever you pick, keep the pair together. A 6 inch gutter fed by 2 by 3 downspouts is the common mistake: the trough holds more water, then the outlets release it at the old rate, and the extra capacity does nothing except delay the overflow.

Slope, hangers and the things that are not quantities

Gutter is hung with a slope of about 1/4 inch per 10 feet toward the downspout. Over a 40 foot run that is a single inch of fall, which is small enough that it has to be set with a chalk line rather than by eye. Too little and the trough holds standing water; too much and it looks wrong against a level fascia.

Hangers go every 24 inches as standard, tightened to 18 inches where snow and ice load the gutter and 16 in heavy snow country. A gutter full of wet snow is carrying far more weight than one full of water, and hangers are the cheapest part of the job.

This calculator does not size for your local rainfall. The figures above assume a typical design storm, and a region that sees short bursts of very heavy rain needs more capacity for the same roof. If you are somewhere with high rainfall intensity, treat a 5 inch result that only just cleared the threshold as a 6 inch job.

Doing this at the same time as the roof

A tear-off is the natural moment for new gutter, because the drip edge and the first course of shingles both tie into it and the gutter usually has to come off anyway. Our roof replacement cost guide covers what the drip edge and the rest of the eave detail cost per linear foot, measured off the same eave run you entered above.