Gutter & Downspout Calculator
Calculate linear feet of gutter, number of downspouts and hangers for any roof.
The gutter and downspout calculator returns linear feet, downspout count, hangers and connectors for any house. Sizing downspouts is the part most DIYers get wrong - you do not space them evenly along the house, you size them to handle the drainage area in your local peak rainfall. A 5-inch K-style gutter handles about 600 sq ft of roof per downspout at 5 in/hr rainfall. In heavy-rain regions (Gulf Coast, parts of Florida) you need either more downspouts or 6-inch gutters. Hangers go every 24 inches; corners and end caps are counted by hand from the roof layout.
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Frequently asked questions
For a 5-inch K-style gutter in average rainfall (5 in/hr peak), one downspout per 600 sq ft of roof drainage area. A 2,000 sq ft roof = 4 downspouts minimum. Bump that up in heavy-rain regions (Gulf Coast, parts of the Pacific Northwest) or downgrade to 5 inch in dry climates. Always have at least 2 downspouts per house even if drainage math suggests one.
5 inch K-style is the residential standard, handles most US homes with 1500-2500 sq ft roof areas in normal rainfall. 6 inch K-style handles 2x the volume - use it for big roofs (over 2,500 sq ft), steep roofs that dump water fast, or any home in heavy-rain regions. 6 inch costs about 30% more but reduces overflow risk and lets you space downspouts farther apart.
24 inches (2 ft) on-center is the standard for hidden hangers in K-style gutters. 18 inches in heavy snow or ice load regions. Half-round gutters with bracket hangers can go to 36 inches on-center because the brackets are stronger. Spiked-and-ferruled gutters (older style) need 18 inch spacing - the spikes pull loose over time.
10 ft sections are the most common pre-fab length sold at home improvement stores. Seamless gutters (continuous rolls run on-site by a gutter installer) can run 40+ ft per section with no joints. Seamless costs 50-100% more than sectional but cuts leak points to zero - worth it if you can find a local installer. Joints in sectional gutter are the #1 leak source.
Yes - at least 2 per downspout (one at the top to direct water from the gutter into the spout, one at the bottom to direct water away from the foundation). Three elbows are common when the downspout has to come around a soffit overhang. Add a splash block or downspout extension at ground level to direct water 4-6 ft away from the foundation.