Rain Gutter and Downspout Sizing Calculator

Design area = plan area x pitch factor x rainfall intensity (in/hr). Split it between the run's downspouts and choose a gutter whose capacity at 1 in/hr covers each share.

1. Measure the plan area for each run. Measure the roof from above (length x depth), not along the slope, and split it by which gutter each part drains into. A gable roof drains half to each eave.
2. Apply the pitch factor. Multiply by 1.00 for flat to 3/12, 1.05 for 4 to 5/12, 1.10 for 6 to 8/12, 1.20 for 9 to 11/12 and 1.30 for 12/12. Steep roofs catch more wind-driven rain.
3. Multiply by rainfall intensity. Use your local 5-minute, 10-year rainfall in inches per hour (often 4 to 8 in the US). The result is the design area at 1 in/hr.
4. Compare each stretch with gutter capacity. Water flows to the nearest downspout, so divide the design area by the number of downspouts on the run. Pick the smallest gutter whose capacity at 1 in/hr covers that share: about 5,520 sq ft for a 5 in K-style and 7,960 for a 6 in.
5. Count downspouts. Each square inch of outlet drains about 1,200 sq ft at 1 in/hr. Divide the design area by 1,200 x the outlet area, and keep downspouts within 40 ft of each other.

This follows the method in SMACNA's Architectural Sheet Metal Manual, simplified for houses. For commercial roofs, long runs or box gutters, check the manual or an engineer.

One square inch of downspout outlet drains about 1,200 sq ft of roof in a 1 in/hr rain. Divide by your rainfall intensity.

A 50 x 40 ft gable roof at 6/12 has two 50 ft eaves, each draining 1,000 sq ft of plan area. 1,000 x 1.10 x 7.6 in/hr = 8,360 sq ft at 1 in/hr per eave. Downspouts: 8,360 / 7,200 (a 2 x 3) rounds up to 2 per eave, 4 in all. With a downspout at each end, each half of the gutter carries about 4,180 sq ft, so a 5 in K-style works. With only one downspout per eave, the whole 8,360 sq ft would overload a 6 in gutter.

A 6 in K-style carries about 45% more water than a 5 in. Upsize when the calculator puts a run above about 80% of a 5 in, when a valley dumps into the run, on roofs steeper than 8/12, on runs over 40 ft with one downspout, or under trees where debris slows the flow. 6 in gutters also take 3 x 4 downspouts, which clog far less.

Where two roof planes meet in a valley, their water lands on a few feet of gutter at high speed and can shoot over the front lip. Count both planes in that run, put a downspout within a few feet of the valley, and add a valley splash guard. Where an upper roof drains onto a lower one, add the upper area to the lower gutter's run.

Price the system in the gutter cost calculator or build a big-box parts list in the Amerimax gutter calculator. Hanger spacing and slope are in the gutter installation cheat sheet. If water already reaches the basement, see the sump pump calculator.