Gutter and Downspout Calculator

Enter your roof edges and wall height to get a gutter shopping list and a downspout size check.

How the calculation works

• Downspouts: one for every 35 feet of gutter, and at least one per run. If the roof area is larger than the downspouts can carry, more are added.
• Downspout capacity: a common rule of thumb is about 600 sq ft of roof per 2" × 3" downspout and 1,200 sq ft per 3" × 4" downspout, for moderate rainfall. Steep roofs catch more wind-driven rain, so the roof area is raised by a pitch factor of 1.0 to 1.3.
• Downspout pipe: wall height plus 2 feet for elbows and the run at the bottom, per downspout.
• Hangers: gutter length divided by the hanger spacing, plus one per run.

Worked example

80 feet of gutter in 2 runs, 10 feet up, on a 1,600 sq ft roof with a 6/12 pitch and 3" × 4" downspouts. The length calls for 3 downspouts. The adjusted roof area of 1,760 sq ft needs only 2, so the answer is 3. That's 36 feet of downspout pipe, 9 elbows, 42 hangers at 24" spacing, and 4 end caps.

Tips

• Slope gutters about 1/4" for every 10 feet toward each downspout.
• Choose 6" gutters instead of 5" for large or steep roofs, or where downpours are heavy. They carry about 50% more water.
• Send water at least 4 to 6 feet from the foundation with extensions or splash blocks.

Frequently asked questions

How many downspouts do I need?

Plan on one downspout for every 30 to 40 feet of gutter, with at least one at the end of each run. Long runs drain better with a downspout at each end.

How far apart should gutter hangers be?

Space hangers every 24 inches in areas with snow or heavy rain, and every 36 inches elsewhere. Put an extra hanger within a foot of each end and on each side of a seam.