Gutter Size Calculator

Five-inch K-style handles most single-family homes when the downspouts are properly spaced — a 5-inch gutter carries the runoff from about 5,520 sq ft of roof at one inch of rain per hour, which is far more than a typical run ever sees. Six-inch is the right call when one gutter run drains a very large or steep roof plane, when a run goes more than about 40 feet without a downspout, or when heavy tree cover means you want the extra clearance and 3x4 downspouts.

Work from area first, then spacing. One downspout drains roughly 100 sq ft of roof per square inch of outlet at one inch of rain per hour — so about 600 sq ft for a 2x3 and 1,200 sq ft for a 3x4, scaled down by your local rainfall intensity. Then check the spacing: one outlet every 30 to 35 feet of gutter, never more than 40 feet apart, and always at least two on a house.

A steeper roof catches wind-driven rain across a larger effective surface and sheds it faster, so the gutter sees a higher peak flow than the ground footprint alone suggests. The standard fix is a multiplier on the footprint: 1.0 for flat to 3-in-12, rising to 1.3 for 12-in-12 and steeper.

The footprint — the area the roof covers when viewed from above. You can pace it out at ground level or take it off a plot plan. The calculator applies the pitch multiplier itself, so measuring along the slope would double-count.

Yes, and more than most people expect. Gutter capacity tables are published at one inch of rain per hour, and the design storm in our Georgia markets is nearly 4 in/hr against about 2.9 in/hr in Cincinnati and South Bend. The same roof needs meaningfully more drainage in Macon than it does in Michiana.

No — treat it as a planning estimate. It sizes one system against a design storm, but it cannot see your valleys, the number of separate roof planes, fascia condition, or where the water has to go after it leaves the downspout. All of those change the specification. We measure them on site as part of a free estimate.