Gutter size calculator: what size gutter does my roof need?
4 in half-round gutter
33.8 gpm at 3.25 in/hr, 39 gpm in Table 1106.6 · 2 downspouts of 2 in round, 2 × 2 or 1 1/2 × 2 1/2 in (30 gpm) · for 50 × 20 ft at 3.25 in/hr
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Enter it the way you measured it: 4.5, 4 1/2,
3/8 or 10' 6".
4 in half-round gutter
33.8 gpm at 3.25 in/hr, 39 gpm in Table 1106.6 · 2 downspouts of 2 in round, 2 × 2 or 1 1/2 × 2 1/2 in (30 gpm)
Source: International Plumbing Code 2021, Section 1106 (Equation 11-1, Tables 1106.3 and 1106.6), and NOAA Atlas 14 rainfall, read October 26, 2026. Read the source
A 1,000 sq ft roof in Atlanta, where NOAA's 100-year one-hour rain is 3.25 in/hr, sheds 33.8 gpm by IPC 2021 Equation 11-1: a 4 in half-round gutter at 1/8 in per ft, rated 39 gpm in Table 1106.6.
A worked example
A 37 ft gutter under a roof 13 1/2 ft deep, with 60 sq ft of dormer wall shedding onto it, 4.2 in/hr of rain, a 1/4 in per ft slope and one downspout needs a 1 1/2 × 2 1/2 in rectangular gutter. 37 × 13.5 is 499.5 sq ft, plus half of the wall, 30 sq ft: 529.5 sq ft × 4.2 × 0.0104 = 23.1 gpm. At 1/4 in per ft Table 1106.6 rates the 1 1/2 × 2 1/2 in gutter for 26 gpm, and one downspout of 2 in round, 2 × 2 or 1 1/2 × 2 1/2 in carries 30.
What size gutter for 1,000 or 2,000 square feet of roof?
At 1/8 in per ft, 1,000 sq ft takes a 4 in half-round gutter in all 15 cities of the table below. At 2,000 sq ft it takes a 5 in one in Atlanta, Chicago or New York and a 6 in one in Houston, Dallas, Miami or Minneapolis, where NOAA's 100-year hourly rain is 3.58 in/hr or more.
How much roof can one downspout drain?
A 3 in round, 2 × 4 or 2 1/2 × 3 in leader carries 92 gpm by IPC Table 1106.3: about 2,720 sq ft of roof at 3.25 in/hr. A 2 × 2 in one carries 30 gpm, about 890 sq ft at the same rain.
Does a steeper roof need a bigger gutter?
Not by the code: the area is the roof projected on the ground (IPC 1106.4), so a 12/12 roof and a 4/12 roof with the same footprint shed the same flow. A wall that sheds onto the roof adds half its area.
What else it is called
Also searched as a rain gutter size calculator, gutter and downspout size calculator or downspout size calculator. The plumbing code calls downspouts vertical leaders and sizes them in Table 1106.3; a gutter's fall toward the outlet is the gutter slope.
Gutter size by city
| City | 100-year, 1-hour rain | Flow per 1,000 sq ft | Half-round gutter, 1,000 sq ft | Half-round gutter, 2,000 sq ft |
|---|---|---|---|---|
| Atlanta, GA | 3.25 in/hr | 33.8 gpm | 4 in | 5 in |
| Houston, TX | 4.85 in/hr | 50.4 gpm | 5 in | 6 in |
| Dallas, TX | 3.69 in/hr | 38.4 gpm | 4 in | 6 in |
| Miami, FL | 5.22 in/hr | 54.3 gpm | 5 in | 6 in |
| Charlotte, NC | 3.19 in/hr | 33.2 gpm | 4 in | 5 in |
| Nashville, TN | 3.13 in/hr | 32.6 gpm | 4 in | 5 in |
| St. Louis, MO | 3.13 in/hr | 32.6 gpm | 4 in | 5 in |
| Chicago, IL | 3.08 in/hr | 32 gpm | 4 in | 5 in |
| Minneapolis, MN | 3.58 in/hr | 37.2 gpm | 4 in | 6 in |
| New York, NY | 2.89 in/hr | 30.1 gpm | 4 in | 5 in |
| Philadelphia, PA | 2.92 in/hr | 30.4 gpm | 4 in | 5 in |
| Boston, MA | 2.62 in/hr | 27.2 gpm | 4 in | 5 in |
| Denver, CO | 2.26 in/hr | 23.5 gpm | 4 in | 5 in |
| Phoenix, AZ | 2.00 in/hr | 20.8 gpm | 4 in | 5 in |
| Los Angeles, CA | 1.63 in/hr | 17 gpm | 4 in | 4 in |
Rain: NOAA Atlas 14 point estimates at each city center, 100-year average recurrence interval, 60-minute duration, read October 26, 2026. Flow by IPC 2021 Equation 11-1 and the smallest half-round gutter of Table 1106.6 at 1/8 in per ft that carries it, for the projected roof area draining to one gutter.
Where the numbers come from
- Flow and sizes: International Plumbing Code 2021, Section 1106, as adopted by Colorado. Equation 11-1: GPM = R × A × 0.0104, R the rainfall in inches per hour and A the roof area in square feet; R is the 100-year hourly rainfall rate (1106.1); half of any vertical wall that sheds onto the roof is added to the projected roof area (1106.4); Table 1106.6 gives gutter capacity by size and slope, e.g. 39 gpm for a 4 in half-round gutter and 110 gpm for a 6 in one at 1/8 in per ft; Table 1106.3 gives leader capacity, e.g. 92 gpm for a 3 in round, 2 × 4 or 2 1/2 × 3 in leader. Read October 26, 2026. IPC 2021 chapter 11 on UpCodes
- Rain: NOAA Atlas 14 point precipitation frequency estimates, Precipitation Frequency Data Server, precipitation intensity, partial duration series, 60-minute duration, 100-year average recurrence interval; at each city center, e.g. Atlanta 3.25 in/hr, Houston 4.85, Chicago 3.08, Denver 2.26. Read October 26, 2026. Pick your own point on the map for your address. NOAA precipitation frequency map
- The rest is arithmetic: roof area = gutter run × roof depth seen from above, plus half the wall; gpm per downspout = gpm ÷ downspouts; the gutter and leader are the smallest in each table that carry it.
Table 1106.6 lists half-round gutters by diameter and rectangular ones by width × depth, at slopes from 1/16 to 1/2 in per ft. K-style gutters, the ones sold as 5 and 6 in, are not in the code table, so the page does not convert to them.
By Javier Miralles · Last updated . How this site is made.
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