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IPC Equation 11-1 flow from plan area, rainfall intensity, and SMACNA pitch factor — then 5" vs 6" K-style and downspout count
This is drainage capacity, not a shingle takeoff and not attic airflow. Plan footprint for squares: Roofing Square & Shingle Bundle Calculator — bring the plan number here, not the sloped surface. Attic NFA: attic ventilation calculator. Why area and rainfall both matter: guide. Overflow vs clog vs slope: troubleshooting. 5" vs 6" K-style: comparison. Roofing index: roofing calculators hub.
Calculator Inputs
Plan (footprint) area draining to this gutter, X/12 pitch for the SMACNA Table 1-1 wind-driven-rain factor, and rainfall intensity. Flow is IPC Equation 11-1: Q = 0.0104 × A × i. Default rainfall is 4 in/hr as a starting point — look up the 100-year hourly rate for anything critical.
Use the bird’s-eye plan area of the slope that dumps into this gutter — not sloped squares and not living-area listing. Do not know the footprint? Measure it on the Roofing Square & Shingle Bundle Calculator (length × width or footprint-area mode) and bring that plan number here. Do not paste the sloped-surface result; drainage uses plan area, then a different pitch factor.
Default: the number you typed is already the catchment for one continuous gutter run. Whole simple gable: a two-slope roof with one gutter on each eave — the tool halves the plan area so you size one eave, not both at once.
Same X/12 as the shingle calculator. Here it does not multiply by √(1+(X/12)²). SMACNA Table 1-1 applies a wind-driven-rain design-area factor: ≤3/12 = 1.00, 4–5/12 = 1.05, 6–8/12 = 1.10, 9–11/12 = 1.20, 12/12+ = 1.30.
IPC 1106.1 wants the 100-year, 1-hour rate from Figures 1106.1(1)–(5) or NOAA Atlas 14 / approved local weather data. 4 in/hr is a common mid-range starting point, not a national code default. Gulf and Florida jobs often sit at 5–7 in/hr; much of the West is closer to 1–3. Check the local rate before you hang metal.
Results
Recommended gutter
—
Enter plan area, pitch, and rainfall intensity
Required flow—
Code equationIPC Eq. 11-1
Plan drainage area—
SMACNA pitch factor—
Effective area—
Gutter size—
Gutter capacity—
Downspouts—
Leader capacity each—
Flow per section—
IPC 2021 §1106.2.1 Equation 11-1: Q (gpm) = 0.0104 × A × i, with A in sq ft and i (R in the code) in in/hr. Pitch factor is SMACNA Architectural Sheet Metal Manual Table 1-1, applied to plan area — not the geometric slope multiplier used for shingle squares. Gutter capacities at 1/8 in/ft from IPC Table 1106.6: 5" semicircle = 74 gpm, 6" = 110 gpm (footnote: single dimensions are semicircle diameters). K-style of the same nominal width is the usual residential trough and is not assumed larger than those tabulated values. Downspouts use IPC Table 1106.3 nearest listed leaders: 2×3 residential treated as 30 gpm (2" / 2×2), 3×4 as 92 gpm (3" / 2×4). IPC 1106.4 adds half of any vertical wall that sheds onto the roof — not in this form. Space downspouts so no continuous run is longer than about 35–40 ft even if capacity math allows fewer. Why area and rainfall both matter: guide. Overflow vs clog vs slope: troubleshooting. 5" vs 6" K-style: comparison.
How to Use This Gutter & Downspout Sizing Calculator
Enter the plan footprint that drains to the gutter you are sizing. If you only have a simple two-slope gable total, switch to whole-gable mode so one eave is half. Type pitch as X/12 (same number as the shingle takeoff). Leave rainfall at 4 in/hr unless you have looked up the 100-year hourly rate. The tool converts area and intensity to gallons per minute with IPC Equation 11-1, applies the SMACNA pitch factor, then recommends 5" or 6" K-style and how many 2×3 or 3×4 downspouts that run needs.
A_eff = A_plan × SMACNA Table 1-1 pitch factor
Q (gpm) = 0.0104 × A_eff × i (IPC Eq. 11-1)
per-section flow = Q ÷ downspout count
5" K if per-section ≤ 74 gpm | 6" K if per-section ≤ 110 gpm (IPC Table 1106.6 @ 1/8 in/ft)
Q (gpm) = 0.0104 × A_eff × i (IPC Eq. 11-1)
per-section flow = Q ÷ downspout count
5" K if per-section ≤ 74 gpm | 6" K if per-section ≤ 110 gpm (IPC Table 1106.6 @ 1/8 in/ft)
This is drainage capacity, not a materials takeoff and not attic airflow. Squares and bundles live in the Roofing Square & Shingle Bundle Calculator — use that page to get plan area, then come back here. Attic NFA is the attic ventilation calculator. Why both area and rainfall matter: guide. Overflow that is not a size problem: troubleshooting. When 6" is worth the extra hangers: 5" vs 6" K-style.
What This Number Is (and Is Not)
The headline is a trough size and a downspout count for one continuous gutter run. Required flow is gallons per minute from the plumbing code’s rainfall conversion, not a linear-feet takeoff and not a carton of hangers. IPC Table 1106.6 lists capacities for semicircular gutters by diameter; residential K-style is the profile you will actually hang, and this tool does not pretend a 5" K-style carries more than the 74 gpm tabulated for a 5" semicircle at 1/8 in/ft.
Assumptions baked into every result
One pitch zone. One rainfall intensity. Area is plan/footprint of the slope draining to this gutter (or half of a simple gable). Pitch factor is SMACNA Table 1-1, not the shingle-calculator geometric multiplier. Gutter slope is the 1/8 in/ft column in Table 1106.6. Downspout count is at least two, then stepped up so per-section flow fits the trough. 2×3 leaders use the conservative 30 gpm line (IPC lists 2" and 2×2 at 30; 2×3 is not a tabulated row). 3×4 leaders use 92 gpm (tabulated for 3" / 2×4 / 2½×3). Vertical walls that shed onto the roof (IPC 1106.4) are not in the form — add half that wall area to the plan number yourself if a tall wall dumps onto this slope.
Plan area, not sloped squares.Rain falls as a vertical catch. Feeding the shingle-calculator’s sloped surface into this tool double-counts slope. Bring the footprint; let SMACNA Table 1-1 apply the smaller wind-driven-rain factor.
When this tool is the wrong tool
Shingle squares, bundles, and felt rolls belong in the roofing square calculator. Attic net free area belongs in the attic ventilation calculator. Common rafter length belongs in the rafter length calculator. Hidden hangers, expansion joints, and fascia condition are not in the gpm math.
Need the plan footprint first?Length × width (or footprint-area mode) on the square calculator. Bring that plan number here — skip the sloped-surface total.
Open Square Calculator →Any trade
Checkout opens here — you stay on this page.
Gutter Drainage Gear
The calculator returns gallons per minute and a 5" or 6" call. A guard, a downspout extender, and a scoop are how that capacity survives leaves and keeps discharge off the foundation.
Amerimax 86670 Snap-in Filter Gutter Guard 25-Pack
Fits 4, 5, and 6 in K-style gutters. Guards do not add capacity — size the trough first, then keep leaves from eating that capacity.
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Roonova Flexible Downspout Extenders 2-Pack with Strainers
Fits 2x3 and 3x4 downspouts. The calculator sizes the spout; the extender is how you keep that discharge off the foundation.
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Amerimax 8300 Gutter Getter Scoop 2-Pack
Flexible scoop that fits K-style troughs. Overflow that looks like a sizing problem is often a clog — clean it before you upsize.
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Run the Drainage Math
Plan area, SMACNA pitch factor, and rainfall intensity. Flow in gpm, 5" vs 6" K-style, downspout count.
Open Gutter Calculator →
Need Plan Area First?
Length × width footprint on the square calculator. Use that plan number here — do not paste sloped squares into drainage math.
Open Roofing Square Calculator →
Roofing Calculators Hub
Shingle takeoff, attic NFA, and gutter flow. Three different jobs on one index.
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