Gutter calculator

Downspouts are the number that decides whether the gutter works

Linear feet of gutter is the easy half of the job. Linear feet is not what fails. What fails is one downspout trying to drain half a roof: the gutter overtops at the far end, water sheets down the fascia, and the soil beside the foundation stays wet.

The sizing rule is simple and it is the one SMACNA uses — one square inch of downspout cross-section per 100 sq ft of drainage area, at about an inch of rain an hour. Work it through and you get the published table exactly:

So a 1,200 sq ft roof on a 6/12 pitch is 1,320 sq ft adjusted, which is three 2 × 3 downspouts. Stepping up to 3 × 4 drops that to two on capacity — but 80 ft of gutter still needs three outlets on the run-length limit below, and the binding limit is the one that counts. Rainfall matters as much as area: flow is area times intensity, so a roof in a region that plans on 7 in/hr needs seven times the outlet of the same roof at 1 in/hr. Use your local peak 5-minute intensity, not the annual average.

There is a second limit that has nothing to do with capacity. Gutter is hung at about 1/4 in of fall per 10 ft, so water has to travel a long way at almost no slope. A run over 60 ft needs at least two outlets whatever the roof area, which is where the 30 ft per outlet default comes from.

Gutter size sets the ceiling

The gutter itself carries far more than one outlet can release, so the downspout is nearly always the binding constraint — but the sizes still have to pair up. A 5 in K-style gutter takes a 2 × 3 downspout and will carry up to about 5,520 sq ft; a 6 in K-style takes a 3 × 4 and carries about 7,960. Half-round holds much less for its nominal size: 2,500 sq ft at 5 in, 3,840 at 6 in.

The practical read: on anything above about 1,500 sq ft of roof, or any roof steeper than 8/12, go to 6 in gutter with 3 × 4 downspouts. The material cost difference is small and it removes both constraints at once.

Sections, hangers and the rest of the order

Stock K-style comes in 10 ft sections; seamless is rolled on site in one piece and priced by the foot, which removes every joint except the corners. Joints are where gutters leak first, so if the run is long and straight, seamless is worth the call-out.

Hangers go at 24 in on centre as a maximum, tightened to 18 in where snow and ice load the gutter. Then count the pieces this calculator does not: an inside or outside mitre at every corner, an end cap at every open end, a drop outlet at every downspout, plus two elbows and a strap for each downspout run. Order the drip edge at the same time — it is what carries water off the deck and into the gutter in the first place.

What the model is, and what the totals hide

The pitch factors are a drainage allowance from a published sizing guide, not geometry. They exist because a steep roof throws water harder and catches more wind-driven rain. They are not the slope multiplier the roofing pages use, and the two are not interchangeable: at 6/12 this page adds 10% where the shingle page adds 11.8%, and the numbers mean different things.

The rainfall scaling is an extension of a table published at one intensity. One square inch of outlet per 100 sq ft reproduces the standard downspout figures at about an inch of rain an hour; folding your local intensity in as a straight multiplier assumes an outlet's duty scales with the rain rate. That is the right shape of the relationship rather than a hydraulic calculation of your particular downspout, and at severe intensities the trough's own capacity and the outlet detail start to matter as much as the cross-section does.

And the intensity is a figure you have to go and get. NOAA's Atlas 14 precipitation frequency tables publish 5-minute rates for US locations, and many jurisdictions name a design intensity in the plumbing code. Nothing here derives one from where you live: the 1 in/hr default is a benchmark to reason from, not a value for your address, and since it multiplies the drainage area directly it is the input with the most leverage on the answer.

Everything else is worked out from totals, and totals hide the corners. The run-length limit is applied to the whole length you enter, so 80 ft made up of two separate 40 ft eaves comes back as three outlets when each eave in fact needs two of its own — four. Hangers are counted the same way, with one end hanger for the job rather than one per run. And the capacity check assumes the roof divides evenly between the gutters, which it does not when one elevation catches most of the roof.

Sections are the length divided by the stock length, with nothing for the lap at each joint or the offcut at a corner; there is no waste field on this page at all. And there is no gutter-size input. The 5 in against 6 in decision above changes what the trough can carry, but the arithmetic here is driven by the downspout you pick and the length of the run, not by the gutter.