Gutter Size Calculator: What Size Gutters and How Many Downspouts

How the calculator works

It runs two published methods and the stricter one wins. The SMACNA method (Sheet Metal and Air Conditioning Contractors' National Association, Architectural Sheet Metal Manual, chapter 1) is the trade standard for sizing gutters. The plumbing-code method is Section 1106 of the International Plumbing Code, which most states adopt for storm drainage.

1. Design area. Rain doesn't fall straight down, so a steep roof catches more than its footprint. SMACNA multiplies the plan area by 1.00 (flat to 3/12), 1.05 (4–5/12), 1.10 (6–8/12), 1.20 (9–11/12) or 1.30 (12/12).
2. Downspouts. One square inch of downspout drains 1,200 ÷ I square feet of design area, where I is the rainfall intensity in inches per hour — the relationship behind SMACNA's city table (7.5 in/hr → 160 sq ft per square inch). We use the downspout's inlet area with SMACNA's ¼-inch allowance (4.80 sq in for a 2×3, 10.31 for a 3×4), so the outlet rather than the pipe sets capacity.
3. Gutter run. No downspout serves more than 50 feet of gutter, SMACNA's practical maximum between ends or expansion joints.
4. Gutter size. For each gutter size, the calculator finds the fewest downspouts at which every section carries its share of the roof. K-style gutters are an irregular shape, so — following SMACNA's instruction for irregular sections — each is sized as the largest rectangle that fits inside it (bottom width × front height: 3⅜ × 3⅝ inches for a 5-inch, 3⅞ × 4⅝ for a 6-inch, from a roll-former's profile sheet) using SMACNA's level-gutter equation. Half-round gutters use SMACNA's published half-round capacities. Both are conservative: real K-style gutters hold more than the inscribed rectangle, and sloped gutters flow faster than level ones.
5. Code check. The plumbing code converts rain to flow with GPM = R × A × 0.0104 (IPC Equation 11-1), using the 100-year, 1-hour rate R, and caps each leader and gutter at its table capacity (IPC Tables 1106.3 and 1106.6). Common residential sizes aren't all in those tables, so each is held to the largest listed size that fits inside it — a 2×3 downspout to the 2 × 2 row (30 gpm), a 3×4 to the 2½ × 3 row (92 gpm), a 5-inch K-style gutter to a 4-inch semicircle (39 gpm at ⅛ in/ft).

Which storm to design for, and why 5 minutes

A gutter fills in seconds, so it overflows in the hardest few minutes of a storm, not over the hour. That is why SMACNA sizes gutters on the 5-minute intensity — the rate rain falls during the heaviest 5 minutes — for a storm expected once in 10 years (its standard column) or once in 100 years (where overflow would do damage, such as over an entrance). The plumbing code instead uses the 100-year, 1-hour rate, which is typically a third to half of the 5-minute figure.

All three numbers come from NOAA Atlas 14, the federal precipitation-frequency study, through NOAA's Precipitation Frequency Data Server (partial-duration series, intensities in inches per hour). SMACNA itself points designers to NOAA's server rather than the older city table printed in its manual. Most homes are built under the International Residential Code, which has no gutter-sizing table at all; the plumbing code and SMACNA are the published methods that do.

Gutter and downspout sizing charts

Square feet of design area (plan area × pitch factor) one downspout drains, by SMACNA's method:

Design rainfall intensity by state

The 5-minute intensity varies more than most gutter guides admit — several-fold between the driest and wettest counties. The map shows every county's value; the table gives each state's median and range. Values are NOAA Atlas 14 point estimates read at each county's population-weighted centre, for 3,144 counties.

Where NOAA Atlas 14 has no numbers

Oregon and Washington are the only states Atlas 14 does not cover. NOAA still points them to NOAA Atlas 2, Volume 10 (1973), which maps 6- and 24-hour depths rather than the short-duration intensities gutters need, so the calculator doesn't invent a value there — it asks you to enter your local design rainfall, which your building department can give you. A few remote Alaska boroughs also have no value at their population centre. Idaho, Montana and Wyoming are covered by Atlas 14 Volume 12. A zip gets the values for the county that holds most of its land; for an exact address, NOAA's Precipitation Frequency Data Server gives the value at any point.

Open data: design rainfall by county (CSV)

The full county table behind this calculator is free to download: design-rainfall-by-county.csv. One row per county with its FIPS code, the point the values were read at, the Atlas 14 volume, 5-minute intensities at 2-, 10-, 25- and 100-year recurrence, the 15-minute 10-year and 1-hour 2-, 10- and 100-year intensities (in/hr), and 24-hour storm depths at 1-, 2-, 10-, 25- and 100-year recurrence (inches, the 24-hour intensity × 24). Counties Atlas 14 doesn't cover are listed with empty values.

It is licensed CC BY 4.0: use it for anything, including commercially, with credit to Yard Drainage Fixes and a link to this page. See the terms. Source: NOAA Atlas 14 via the NOAA Hydrometeorological Design Studies Center, read in September 2026; county points from the US Census Bureau's 2020 centres of population; zips from the Census 2020 ZCTA-to-county relationship file.

Where each downspout should discharge

Gutter guides stop at the count. For drainage, the count is where the problem starts: each outlet concentrates hundreds of gallons at one point in a storm.

• Away from the wall. Extend every outlet 4 to 6 feet onto ground that slopes away — at least 5 feet where soils are expansive. A downspout ending at the foundation is the most common cause of water pooling near the foundation.
• Not all to one corner. Spread outlets along both eaves. Routing several downspouts to the same low corner creates the pond you were trying to avoid.
• Somewhere that can take it. A buried line to daylight, a catch basin, or a dry well sized for the outlet's gallons. The per-outlet figure above feeds the dry well calculator directly.

What this calculator does not tell you

It assumes clean gutters hung so water actually enters them, outlets at the ends of the sections they drain, and roof area spread evenly along the gutter. Valleys that dump onto one short stretch of gutter, upper roofs draining onto lower ones, and ice dams all need more capacity at that spot than an even split suggests. The county value is read at one point; in mountainous counties a specific address can differ, so use NOAA's server for a precise site. And it says nothing about whether the ground can absorb what the outlets deliver — that is the yard question the rest of this site exists for.

Sources

• NOAA Hydrometeorological Design Studies Center, NOAA Atlas 14 Precipitation-Frequency Atlas of the United States, via the Precipitation Frequency Data Server; Oregon and Washington status from NOAA's state information pages.
• SMACNA, Architectural Sheet Metal Manual, 7th ed., chapter 1 (Tables 1-1 to 1-4, Chart 1-1), and SMACNA's Downspout & Gutter Sizing Calculator notes on using NOAA 5-minute intensities.
• International Code Council, 2021 International Plumbing Code, §1106 (Equation 11-1, Tables 1106.3 and 1106.6), and 2021 International Residential Code, §R801.3, read on codes.iccsafe.org.
• K&M Sheet Metal, “K Style ‘Ogee’ Gutter” dimensions sheet (2016) for 5-, 6- and 7-inch K-style profiles.
• US Census Bureau, 2020 county centres of population and 2020 ZCTA-to-county relationship file.

Related

• Roof runoff calculator — the gallons the whole roof sheds in a storm, with your zip's storm depths
• Downspout drainage problems — the symptoms of outlets in the wrong place
• Gutter drainage: where downspout water should go — extensions, buried lines, and pop-up emitters
• Dry well calculator — size storage for one outlet
• North Carolina rainfall design storms — the NC numbers in more depth