Gutter & Downspout Sizing Calculator
Size your gutters and count your downspouts the way sheet-metal pros do: roof area, pitch and the 5-minute rainfall rate a 10-year storm brings to your area.
A real person answers. Free estimate, no obligation.
Data: NOAA Atlas 14 point precipitation frequency (5-minute, 10-year) · Method: SMACNA roof-area gutter sizing · Updated September 2026
How gutter sizing works
Effective roof area
Start with the roof footprint, the length times width of the house as seen from above. A steeper roof catches wind-driven rain and sheds it faster, so the footprint is multiplied by a pitch factor: 1.0 for a low slope, up to 1.3 for a 12:12 roof.
Local rainfall intensity
Gutters are sized for the worst short burst, not a whole day of rain. The standard figure is the 5-minute rainfall rate of a 10-year storm, in inches per hour, from NOAA Atlas 14. It runs from about 3 inches per hour on the Pacific coast to 10 or more on the Gulf Coast. On a town page the tool uses that town’s own Atlas 14 value.
Matching gutters and downspouts
In the SMACNA roof-area method a 5-inch K-style gutter handles about 5,500 square feet of roof at 1 inch per hour and a 6-inch about 8,000, so the effective area times the rainfall rate tells you which one fits. A 2 by 3 inch downspout handles about 600 square feet of roof in an 8 inch per hour cloudburst and a 3 by 4 about twice that; the tool scales those with your rate and adds a downspout at least every 35 feet of gutter.
What the tool assumes
A simple roof that drains to two eave-side gutters, each with the same share of the roof. Valleys, dormers, an inside corner where two roofs meet, or a section that drains onto a lower roof all concentrate water and need a pro’s eye.
Common questions
Most modest homes in moderate-rain areas work with 5-inch K-style gutters. Large roofs, steep pitches and heavy-rain regions like the Gulf Coast, Florida and the Plains push toward 6-inch, which carries about 40 percent more water and takes the larger 3 by 4 downspouts.
At least one every 30 to 40 feet of gutter, and more when the roof area or the rainfall rate is high. A typical 40 by 30 foot house ends up with four: one near each corner.
Those are the inside dimensions in inches of the two common rectangular downspouts. A 3 by 4 carries roughly twice the water of a 2 by 3 and clogs less, which is why pros pair it with 6-inch gutters and use it in heavy-rain areas even on 5-inch gutters.
A steep roof sheds water faster and catches more wind-driven rain, so it delivers water to the gutter in a harder rush. The pitch factor accounts for that; on very steep roofs water can also overshoot the gutter, which a pro handles with the hanging height, a drip edge or splash guards.
The 5-minute rate of a 10-year storm in inches per hour. NOAA Atlas 14 publishes it for any address. Town pages on this site use their own value; the standalone tool offers typical values by region.
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