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.
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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