Gutter & Downspout Size Calculator (Roof Runoff, Gutter Capacity, Number of Downspouts)
Check whether a gutter can carry the runoff from your roof in a design storm, and how many downspouts it needs. Uses the USDA NRCS gutter and downspout capacity table and your local rainfall intensity.
Formula
Runoff Q (cfs) = roof area (sq ft) × rainfall intensity (in/hr) ÷ 43,200 (12 in/ft × 3,600 s/hr); gpm = cfs × 448.8. The gutter is adequate when Q ≤ its NRCS PA-1 capacity at your slope. Downspouts = the larger of ⌈Q ÷ one downspout's PA-1 capacity⌉ and ⌈run length ÷ max length per downspout⌉. Max roof area for the gutter = capacity × 43,200 ÷ intensity.
Worked example
Inputs: Roof area draining to this gutter (plan view, not slope length) = 1200 sq ft; Design rainfall intensity (5-minute depth from NOAA Atlas 14 × 12) = 6 in/hr; Gutter = alum5; Gutter slope = 1/16; Downspout = 2x3; Gutter run length = 40 ft; Max gutter length per downspout (SMACNA practical max 50 ft) = 50 ft.
| Runoff cfs | 0.167 |
|---|---|
| Runoff gpm | 74.8 |
| Gutter capacity cfs | 0.18 |
| Gutter capacity used pct | 93 |
| Max roof area for this gutter sq ft | 1,296 |
| Downspout capacity cfs | 0.11 |
| Downspouts for flow | 2 |
| Downspouts for length | 1 |
| Downspouts needed | 2 |
This gutter carries the design runoff. Pitch it toward each outlet and keep downspouts at least the size listed.
FAQ
What size gutter do I need?
Multiply the roof area that drains to the gutter by your design rainfall intensity and divide by 43,200 to get cubic feet per second, then pick a gutter whose capacity is at least that. At 6 in/hr (0.5 in in 5 minutes, the NRCS PA-1 design storm), a 5 in aluminum K-style gutter at 1/16 in/ft (0.18 cfs) handles about 1,296 sq ft of roof; a 6 in aluminum gutter (0.30 cfs) handles about 2,160 sq ft.
How many downspouts do I need?
Divide the runoff by one downspout's capacity and round up, and use at least one per 50 ft of gutter (SMACNA's practical maximum because of thermal expansion). For 1,200 sq ft at 6 in/hr (0.167 cfs) on a 5 in aluminum gutter, 2 × 3 in downspouts (0.11 cfs each in PA-1) means 2 downspouts; 3 × 4 in (0.22 cfs) means 1.
What rainfall intensity should I use?
Look up the 5-minute rainfall depth for your address on the NOAA Atlas 14 server and multiply by 12. Checked on 2026-10-20 for the 10-year storm: West Palm Beach 0.944 in (about 11.3 in/hr), Miami 0.903 in (about 10.8 in/hr), Chicago 0.619 in (about 7.4 in/hr). South Florida storms need much bigger gutters than the 6 in/hr default.
Does roof pitch matter?
This calculator uses the plan (horizontal) area, which is what catches vertical rain. Some gutter guides multiply the area by a roof-pitch factor (for example 1.1 for a 6/12 roof); if you follow one, enter the adjusted area.
Does a steeper gutter carry more water?
Yes. In the PA-1 table a 5 in aluminum gutter carries 0.18 cfs at 1/16 in/ft, 0.25 cfs at 1/8 in/ft and 0.35 cfs at 1/4 in/ft. PA-1 cautions that 1/32 and 1/64 in/ft slopes are harder to install.
Sources
- USDA NRCS Design Guide No. PA-1, Roof Gutters & Downspouts (rev. 12/2019): maximum gutter capacity in cfs by gutter style, size and slope, and the orifice discharge of compatible downspouts
- SMACNA Downspout & Gutter Calculator, notes (based on the Architectural Sheet Metal Manual, 7th ed.): 50 ft is a practical maximum gutter length served by one downspout; downspouts must not exceed the gutter bottom width
- NOAA Atlas 14 Precipitation Frequency Data Server (look up the 5-minute rainfall depth for your location; intensity in in/hr = depth × 12)
- Geometry (volume/area formulas); no external constant needed
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Last reviewed 2026-10-20.