Gutter Calculator
Size gutters and downspouts for a roof from its area, pitch and local rainfall, and count the linear feet, downspouts, hangers and cost for the job. It follows the SMACNA sizing method: design roof area by pitch, 5-minute rainfall intensity, downspout area and the gutter's capacity.
Your numbers
Length of the eave the gutter hangs on.
Measured flat, from the eave to the ridge (half the building width on a simple gable), not up the slope.
Rise per 12 in of run: 6 for a 6/12 roof.
The heaviest 5 minutes of rain you design for, in inches per hour. Find yours in NOAA Atlas 14 (precipitation intensity, 5-min duration) or the city table below.
36 in on center at most, 18 in where ice and snow last (Berger). Use the hanger maker's spacing if it is closer.
For the feet of downspout pipe.
Your layout: often two to bring the pipe back to the wall and one at the foot.
Two for a straight run with both ends open; a corner piece replaces two.
Gutter size
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- Downspouts
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- Gutter
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- Design roof area
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- Hangers
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- End caps, outlets, elbows
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The calculation runs in your browser and ParrotNotes does not store your numbers. They sit in the page address so you can bookmark or share the result.
How gutters and downspouts are sized
The SMACNA Architectural Sheet Metal Manual sizes roof drainage from three things: how much roof drains to each gutter, how hard it rains in a short burst, and how much water a downspout and a gutter can carry. Rain does not fall straight down, so a pitched roof catches more than its plan area, but SMACNA warns that using the full sloped area oversizes the system. It multiplies the plan area by a pitch factor instead: 1.00 up to 3/12, 1.05 for 4/12 to 5/12, 1.10 for 6/12 to 8/12, 1.20 for 9/12 to 11/12 and 1.30 at 12/12.
Each square inch of downspout drains 1,200 ft² of that design area at 1 inch of rain per hour, so the drainage per downspout is its cross-section × 1,200 ÷ your intensity. Gutters are rated the same way, by the roof area they drain at 1 in/hr. The calculator counts the downspouts each run needs (at least one per 50 ft of gutter), checks the gutter against each downspout's share of the run and picks the smallest gutter that carries it.
Pitch factors and gutter capacity
The design area is the plan area times the factor for the roof's pitch (SMACNA Table 1-1, as reproduced by Berger).
| Roof pitch | Factor |
|---|---|
| Flat to 3/12 | 1 |
| 4/12 to 5/12 | 1.05 |
| 6/12 to 8/12 | 1.1 |
| 9/12 to 11/12 | 1.2 |
| 12/12 and steeper | 1.3 |
Design roof area each gutter size drains per downspout, level gutter. The 1 in/hr column is the rating; the others divide it by the rainfall.
| Gutter | 1 in/hr | 4 in/hr | 6 in/hr | 8 in/hr |
|---|---|---|---|---|
| 5 in K-style | 5,520 ft² | 1,380 | 920 | 690 |
| 6 in K-style | 7,960 ft² | 1,990 | 1,327 | 995 |
| 5 in half-round | 2,500 ft² | 625 | 417 | 313 |
| 6 in half-round | 3,840 ft² | 960 | 640 | 480 |
A worked example
A 40 × 30 ft house with a 6/12 gable roof and gutters on both long sides: each eave is 40 ft with 15 ft of roof behind it, 600 ft² of plan area and 660 ft² of design area. In an area with 7 in/hr of 5-minute rain, a 3 × 4 in downspout drains 12 × 1,200 ÷ 7 = 2,057 ft² and a 5 in K-style gutter 5,520 ÷ 7 = 789 ft², so one downspout and a 5 in K-style serve each side.
The job is 80 ft of gutter, 2 downspouts, 4 end caps, 2 outlets and, at three elbows each, 6 elbows. At 36 in spacing each 40 ft run takes 14 hangers, 28 in all. With a 10 ft eave height that is 20 ft of downspout pipe.
Finding your rainfall intensity
Use the 5-minute intensity, in inches per hour. SMACNA's table lists a 10-year storm and a 100-year storm for each city; the 100-year figure is the cautious choice for a building where an overflow would do damage. For a current local value, NOAA's Atlas 14 precipitation frequency data server gives precipitation intensity by duration and return period for a point on the map.
The city table below comes from Berger's reproduction of the SMACNA table, based on records through 1978. It is a useful check; local code may name its own design storm.
5-minute rainfall intensity by city
In inches per hour, from Berger's reproduction of the SMACNA table. Check NOAA Atlas 14 for your exact location.
| City | 10-year | 100-year |
|---|---|---|
| Atlanta, GA | 7.3 | 9.9 |
| Boston, MA | 5.3 | 7.2 |
| Chicago, IL | 6.8 | 9.3 |
| Dallas, TX | 7.6 | 10.5 |
| Denver, CO | 5.7 | 9.1 |
| Houston, TX | 8.2 | 10.8 |
| Los Angeles, CA | 4.9 | 6.7 |
| Miami, FL | 7.7 | 9.8 |
| Minneapolis, MN | 7 | 10 |
| New York City, NY | 6.7 | 9.2 |
| Phoenix, AZ | 5.6 | 8.8 |
| Seattle, WA | 2.1 | 3.3 |
For roofing and gutter sales: what to write down
On the walk-around, note each eave length, the roof pitch on each face, the eave height, where downspouts can and cannot go (doors, walkways, the neighbor's lot), where the water has to drain to, the material and color, and any fascia rot or old hangers to replace. Read the numbers back before you leave, so the quote matches the house.
Sources, each read on 2026-10-30. Results are estimates; local code, the gutter maker's data and your rainfall data govern:
- Berger Building Products: Proper Gutter and Downspout Sizing, technical appendix A3 to A10 (SMACNA pitch factors, rainfall table, downspout areas, half-round gutter capacities, 50 ft per downspout, hanger spacing)
- SMACNA: Downspout & Gutter Sizing Calculator (ASMM method, 5-minute intensity for a 10-year or 100-year storm, NOAA data)
- NOAA Atlas 14 Precipitation Frequency Data Server (local rainfall intensity)
- This Old House: Sizes of Gutters (K-style and half-round ratings, pitch factors)
Quoting the roof too? The roofing calculator works out the squares and bundles from the same measurements, and the siding calculator the walls.
More for roofing and home services sales: Roofing sales training, Home improvement sales, Building materials sales.
The gutter job is measured on the roof walk
Eave lengths, where the downspouts can go, the drainage the homeowner worries about and the color they want all come up on the walk-around. Record the visit in ParrotNotes and get a transcript, a summary and the action items, so the quote matches what you measured.
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