Gutter Sizing Calculator
The right K-style gutter size, downspout count and size, and total gutter length — from your roof footprint, pitch, and local rainfall, using the standard adjusted-square-footage method.
Gutter sizing has a real method behind it, and most homeowners guess. The honest answer is the adjusted-square-footage approach the trades use: take your roof's footprint, multiply by a pitch factor for how steeply it sheds water, and multiply by your region's rainfall intensity. Compare that adjusted area to K-style capacity — about 5,520 sq ft for a 5-inch gutter and 7,960 for a 6-inch — and you have your size.
This reproduces the standard worked example used by This Old House: a 1,000 sq ft Chicago roof at a 6:12 pitch in 6.8 in/hr rain works out to 7,480 adjusted sq ft, which calls for a 6-inch gutter. Size the roof itself with the Roof Shingle Calculator and the Square Footage Calculator, and turn linear feet into a price with the Cost per Square Foot Calculator. Keeping gutters sized and clear is part of the humid-climate maintenance guide.
FAQ
- What size gutters do I need, 5 inch or 6 inch?
- Multiply your roof footprint by a pitch factor and by your local rainfall intensity (inches per hour) to get the adjusted square footage. A 5-inch K-style gutter handles up to about 5,520 sq ft; a 6-inch handles up to about 7,960. Above that, split the run or go larger.
- How many downspouts do I need?
- The standard rule is one downspout for every 20 to 40 feet of gutter — 35 feet is a common default. This calculator divides your eave length by that spacing and rounds up, with a minimum of one. Steeper roofs and heavier rain are handled by stepping up the gutter and downspout size, not by piling on downspouts.
- Why does roof pitch matter for gutters?
- A steeper roof catches more wind-driven rain, so the method multiplies the footprint by a pitch factor: 1.0 for flat to 3:12, up to 1.3 for 12:12 and steeper. That adjusted area is what the gutter capacity is compared against.
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