Q (gpm) = horizontally projected roof area × rainfall intensity × 0.0104. A correctly sized gutter hung dead level still overflows, and the code table is the proof.
0.0104gpm per sq ft per in/hrDefinitional, not a survey. One inch per hour on one square foot is 1/12 cu ft/hr. NIST SP 811 Appendix B.9: a cubic foot is 7.4805 US gallons. (1/12 × 7.4805) / 60 = 0.010390, published and run as 0.0104.
The short answer
A 2,000 sq ft projected roof at 3 in/hr produces 62.4 gpm. A 6-inch semicircular gutter at 1/8 in/ft is rated 110 gpm (IPC Table 1106.6). Two 3-inch round leaders at 92 gpm each carry it. A 2×3 rectangular downspout (the size on most American houses) is not rated in Table 1106.3. This tool brackets it rather than inventing a number. The 3 in/hr intensity is a placeholder. Read yours from NOAA Atlas 14.
Flow first, then the profile
Projected area, intensity, gutter rating, downspout count. Nothing is emailed.
—Flow this roof produces
—Gutter capacity
—Downspout capacity
—Does this combination carry it?
What this assumed—
Residential K-style gutters are matched by nominal size only. IPC Chapter 11 sizes storm drainage for buildings under the plumbing code; residential gutters are usually never sized against it. HyreRoof does not install gutters.
Gutters are sized by water, not by house
Gutter sizing is not a function of how big the house is. It is a function of how much water arrives at the gutter in the worst few minutes of a storm, which depends on three things: the roof area draining to that run, the pitch of that roof, and the local rainfall intensity.
Pitch matters more than most people expect, and not for the reason they assume. A steeper roof does not collect more rain, but it delivers it faster and with more horizontal momentum, so the effective load on the gutter is higher. Sizing methods apply a pitch factor for exactly this reason.
HyreRoof analysis: the input that varies most between houses is rainfall intensity, and it is regional. A gutter sized adequately in Seattle, which has many wet days but low peak intensity, can be badly undersized on the Gulf Coast, where an inch can fall in fifteen minutes. Steady drizzle is not what overwhelms a gutter; short violent bursts are.
Profile and size
Profile
Relative capacity
Where it fits
Notes
5 in K-style
Baseline
The default on most American homes
Adequate for modest roof areas at moderate rainfall intensity
6 in K-style
About 1.9× a 5 in
Large roof planes, steep pitches, high-intensity rainfall
Takes a 3×4 in downspout, which carries far more than a 2×3 in
5 in half-round
About 0.8× a 5 in K
Period and heritage properties
Less capacity at the same nominal size: the cross-section is smaller
6 in half-round
About 1.4× a 5 in K
Heritage properties with large roof planes
The half-round option when 5 in is not enough
Nominal size and profile together determine capacity. A half-round and a K-style of the same stated size are not equivalent.
The downspout matters as much as the gutter. A correctly sized gutter feeding an undersized downspout simply backs up and overtops at the far end of the run.
What overflows a gutter that is nominally big enough
Relative carrying capacity by gutter profile. Indexed to a 5 inch K-style.HyreRoof presentation of standard gutter cross-sections.
Cause
What happens
Fix
Too few downspouts
Water runs the length of the gutter and overtops before it reaches an outlet
Add outlets. Spacing matters more than gutter size on long runs
Undersized downspouts
The gutter fills faster than the outlet drains
A 3×4 in downspout carries substantially more than a 2×3 in
Valleys concentrating flow
A valley delivers two roof planes to one short section of gutter
Add an outlet directly below the valley, or a splash guard
Insufficient fall
Water sits rather than running to the outlet, and debris settles in the standing water
Around 1/4 inch of fall per 10 feet toward each outlet
Debris
Effective cross-section reduced, then blocked entirely at the outlet
Clearing, and trimming what overhangs the roof
Pitch underestimated
A steep roof throws water over the front lip rather than into the trough
Larger profile, or gutter positioned to catch the trajectory
Most gutter overflow is a drainage-path problem rather than a capacity problem.
If a gutter overflows in one specific place every time, that is almost never capacity. It is a valley, an outlet spacing issue, or a fall problem at that point.
Why undersized gutters cost more than gutters
Fascia and soffit rot
Water overtopping at the back of the gutter runs down the fascia. This is the most common and most expensive consequence, because it is structural timber behind a painted board.
Foundation and basement water
A gutter that overflows deposits concentrated water at the base of the wall, precisely what the drainage system exists to prevent.
Ice dams get worse
Standing water in a gutter freezes and forms the dam that subsequent meltwater backs up behind. Adequate capacity and fall reduce, though do not eliminate, the problem.
Erosion and hardscape staining
Concentrated discharge at a single overflow point undermines planting and stains paving.
Gutter guards do not add capacity
They reduce debris entry and in most designs slightly reduce effective capacity. A guard on an undersized gutter is an undersized gutter that is harder to inspect.
What this calculator does not model
Your actual local rainfall intensity
It works from a selectable intensity band rather than your specific location. For a design that matters, the local 5-minute or 10-minute intensity figure from a rainfall atlas is the input to use.
Downspout placement
It sizes the trough. Outlet spacing is frequently the binding constraint on a long run, and no capacity figure captures it.
Where the water goes afterwards
Discharging a correctly sized system at the base of a wall solves the gutter problem and creates a foundation one. Extensions and drainage are a separate design.
Snow and ice loading
Gutters in cold climates carry structural loads this calculation does not consider, and hanger spacing matters more there than profile does.
The area draining into your gutter is larger than your roof
Sizing a gutter to the roof alone is the single most common sizing error on houses with an upper storey, a dormer wall or an adjoining taller building, and the plumbing code is explicit about it.
Illustrative. The two-opposite-walls case adding nothing is the detail that shows the rule is about wind direction, not about surface area.IPC 2018 §1106.4, Vertical walls, International Code Council. codes.iccsafe.org returned HTTP 403 to direct automated retrieval of this section page on 5 September 2026. The provision and its wall-configuration cases were read through the ICC public search surface and are recorded that way. These appended sections do not reproduce Table 1106.6: the capacities used here are the ones the calculator above states on its own face.
What the code says to add
IPC 2018 §1106.4, Vertical walls: "In sizing roof drains and storm drainage piping, one-half of the area of any vertical wall that diverts rainwater to the roof shall be added to the projected roof area for inclusion in calculating the required size of vertical conductors, leaders and horizontal storm drainage piping."
The reason is wind. Rain does not fall vertically in the weather that matters for sizing; it arrives at an angle, strikes the wall, and runs down it onto the roof below. The code’s answer is to count half of that wall as if it were roof, because on average that is roughly how much of it contributes.
The code also handles the more complicated geometries. For two adjacent walls of equal height, 35 percent of the total wall areas is added. For two adjacent walls of unequal height, 35 percent of the common height plus 50 percent of the remaining height of the taller. Two opposite walls of the same height add nothing, because whichever way the wind blows, one of them is sheltered and the pair cannot both contribute at once.
How this was read. codes.iccsafe.org returned HTTP 403 to direct automated retrieval of this section page on 5 September 2026. The provision and its wall-configuration cases were read through the ICC public search surface and are recorded that way. These appended sections do not reproduce Table 1106.6: the capacities used here are the ones the calculator above states on its own face.
Model code, locally adopted. Model code. Adoption and amendment are local, and the edition your jurisdiction adopted may differ. Confirm with your building department before relying on any of it.
Why this catches people out
A two-storey house with a single-storey rear extension is the classic case. The extension roof might be 300 square feet. The wall of the main house above it might be another 300. The code says to size for 450, which is half as much water again as the roof area suggests, and the gutter on that extension is invariably the one that overflows.
Attached garages, dormer cheeks, chimney stacks and any wall between two roof planes at different heights all do the same thing at smaller scale. So does a neighbouring building on a boundary, which is nobody’s roof and is still shedding onto yours.
The calculator above takes an area, and the honest input for that field on a house like this is the projected roof area plus the wall contribution, not the roof area. That single change moves many gutters up a profile.
Get your own rainfall intensity, and get the right duration
The intensity field is the input that most changes the answer and the one most often left on a placeholder. It is also freely available for your exact coordinates.
Drainage sizing runs on a rainfall intensity in inches per hour for a stated duration and a stated recurrence interval. The federal source for that in the United States is NOAA Atlas 14, distributed through the National Weather Service’s Precipitation Frequency Data Server, which publishes point estimates by latitude and longitude for a range of durations and average recurrence intervals, with the supporting documentation alongside.
The duration matters as much as the number. A gutter is a small, fast system with a few minutes of storage in it. A short-duration intensity: the kind of cloudburst that fills a trough in five minutes, is what overwhelms it, not a long steady soak that delivers more total water.
Sizing from an hourly average when the binding case is a five- or ten-minute burst is how a system that passes the arithmetic still runs over the front edge twice a summer.
And the recurrence interval is a decision, not a fact. Sizing for a 100-year event costs a little more in profile and outlet count and buys you a system that essentially never overtops. Sizing for a 10-year event is cheaper and means accepting overflow on the worst day of most decades. Which is right depends on what is under the gutter: a lawn, or a door threshold, or a basement light well.
What we do not do. HyreRoof does not restate NOAA’s numbers for your location, because your location is a lookup and restating a national figure would be worse than the lookup. The calculator takes the intensity as an input for exactly that reason. NOAA has also been developing a successor product, Atlas 15; where your jurisdiction has adopted design values, the jurisdiction’s values are the ones that govern.
Model code, locally adopted. Model code. Adoption and amendment are local, and the edition your jurisdiction adopted may differ. Confirm with your building department before relying on any of it.
The trough is rarely what fails. These are.
A gutter is a system: intake at the roof edge, conveyance along the trough, discharge through the outlets, and somewhere for it to go. The trough is the part people size and the part that fails least.
Illustrative. Outlets gate the system. Trough size does not rescue a blocked one.HyreRoof illustration. Not a measurement.
Component
How it fails
What its absence or failure means
Outlet count and spacing
Too few outlets on a long run, or all of them at one end. Water has to travel the length of the trough to leave, and the trough fills from the far end first.
The binding constraint on most residential runs. A correctly sized trough with one outlet at forty feet will overtop at the far end while the outlet is still running below capacity.
Outlet condition
Leaf mat and grit form a plug at the drop. It is the narrowest point in the system and the one nobody can see from below.
A blocked outlet reduces the whole run to whatever the remaining outlets can take. No trough size compensates. This is a maintenance problem masquerading as a sizing problem.
Fall along the trough
Too little and water stands; too much and it runs past the outlet and over the end. Hangers sagging over years change it after installation.
Standing water in a gutter is the ice-dam seed in cold climates and the mosquito habitat everywhere else. Level gutters are a look, not a design.
Back edge and the drip edge above it
Water tracks back under the shingle edge and behind the gutter instead of into it. The code’s answer is edge metal: "A drip edge shall be provided at eaves and rake edges of shingle roofs", which "shall extend not less than 1/4 inch (6.4 mm) below the roof sheathing."
This is the failure that rots fascia and soffit, and it is invisible until the paint fails. It is also the cheapest of all of these to prevent and impossible to retrofit without lifting the covering.
Discharge point
A correctly sized system delivering concentrated water to the base of a wall.
You have solved the gutter problem and created a foundation one. Extensions, splash blocks or a drainage connection are a separate design and are not optional in most soils.
Guards and screens
Debris bridges the mesh, or heavy rain sheets over the top of a solid-cover design instead of into it.
Guards reduce debris entry and in most designs slightly reduce effective capacity. A guard on an undersized gutter is an undersized gutter that is now harder to inspect.
Secondary drainage, where the roof is enclosed
Nowhere for water to go if the primary path blocks. Unless roofs are sloped to drain over roof edges, roof drains shall be installed at each low point of the roof. And where water can be trapped: "secondary emergency overflow roof drains or scuppers shall be provided where the roof perimeter construction extends above the roof"
On a parapeted or enclosed low-slope roof this is a structural safety provision, not a nuisance one, trapped water is a load the roof was not designed for. It is a code requirement, not a nicety.
Ordered roughly by how often each is the actual cause of an overflow that gets blamed on gutter size.
HyreRoof analysis, built on the code provisions quoted. We hold no dataset of gutter failures, do not install or clean gutters, and take no fee from anyone who does.
What a gutter specification should say
Gutters are usually quoted by the foot with a colour. Here is what the line should contain instead.
The drained area, with the wall contribution stated separately
Projected roof area, plus the sidewall addition, with the arithmetic shown. On a house with an upper storey shedding onto a lower roof this is the difference between the right profile and the wrong one.
The intensity used, with its duration and recurrence interval
Not "for heavy rain". A number, a duration and a return period, from NOAA Atlas 14 for your coordinates or from your jurisdiction’s adopted design values.
Outlet count and spacing, not just the total
Where each downspout goes and how far apart they are. This is the constraint that actually binds and the one most often left to whatever is convenient at the corners.
The fall, and the hanger spacing that holds it
A stated fall toward each outlet, and hangers close enough to keep it, closer in snow country, where the trough is carrying a structural load as well as water.
Where the water goes after the downspout
Splash block, extension, buried line or a connection. A specification that ends at the bottom of the downspout has stopped one foot short of the problem it exists to solve.
Do not accept level gutters as a design choice
A gutter with no fall holds water permanently. It is chosen because it looks right against a fascia line, and it is the starting condition for ice and for corrosion.
Do not size from the footprint of the house
The footprint is not the drained area for any run, and on a multi-plane roof each run drains a different part of it. Size each run on what actually flows into it.
Do not treat a guard as extra capacity
It is not, in any design we are aware of. If a guard is being offered as the fix for an overflowing gutter, the diagnosis has been skipped.
The part of this job that actually hurts people
Gutter cleaning is the reason a great many homeowners end up on a ladder at the eave two or three times a year, and ladder falls are a leading cause of serious home injury. It is worth being blunt about that, because gutter work does not feel like roof work and the height is the same.
What the federal standard requires of a trained worker. 29 CFR 1926.501(b): "Each employee engaged in residential construction activities 6 feet (1.8 m) or more above lower levels shall be protected by guardrail systems, safety net system, or personal fall arrest system unless another provision in paragraph (b) of this section provides for an alternative fall protection measure."
How to read that as a homeowner. HyreRoof position: OSHA regulates employers, not homeowners. But the threshold tells you something a homeowner should act on. The federal government requires a trained worker with equipment and a rescue plan to be protected above six feet. "Get up there and have a look" is advice that asks an untrained person to do, unprotected, what a professional may not do unprotected. Falls from height are the leading cause of death in construction. Inspect from the ground, from a window, from a ladder at the eave, or with a drone or a photograph, not from the roof.
Our note on how we read it. osha.gov returned HTTP 403 to automated retrieval on 5 September 2026. The text below was read at the Cornell LII mirror of 29 CFR 1926.501 on that date.
The practical version. Work from the ladder with your feet on the ladder and your belt buckle inside the rails, move the ladder rather than reaching, and never step from a ladder onto a roof. Better still, use a gutter scoop on a pole from the ground, or a hose attachment, or pay someone whose employer is required to protect them. The saving on a gutter clean is not worth what a fall costs.
And this is not the assembly reference. NRCA publishes The NRCA Roofing Manual, the trade’s reference for assembly detailing. It is a purchased document rather than a free publication, so this site does not quote from it and describes only what is publicly and consistently stated about its role. The controlling text is the manual itself, and a competent contractor has a copy.
The vocabulary, once
Horizontally projected area
The footprint of the roof plane as seen from directly above, not its sloped surface. Rain falls vertically for drainage purposes, so the projection is the correct area, which is why gutter sizing does not use the slope factor that material take-offs do.
Sidewall contribution
The share of an adjoining vertical wall that the plumbing code adds to the drained area, because wind-driven rain strikes the wall and runs down onto the roof below. Half of one wall; 35 percent of two adjacent walls of equal height; nothing for two opposite walls of equal height.
Rainfall intensity
Rate of rainfall in inches per hour for a stated duration and average recurrence interval. Published for US coordinates by NOAA Atlas 14 through the National Weather Service. The duration matters as much as the number, because gutters fail to short bursts rather than to long soaks.
Average recurrence interval
The 10-year, 25-year or 100-year label on a design storm. It is a probability statement, not a schedule: a 100-year intensity has roughly a one percent chance in any given year, and can occur twice in consecutive years without anything being wrong.
Outlet or drop
The opening from the trough into the downspout. The narrowest point in the system, the first thing to block, and usually the constraint that actually decides whether a run overflows.
Leader or conductor
The plumbing code’s terms for the vertical pipe that carries water down from the roof drainage, what most people call the downspout.
Secondary or emergency overflow drainage
A second path out, required where the roof perimeter construction can trap water if the primary drains block. On a parapeted roof it is a structural safety provision: trapped water is a load nobody designed for.
Fall
The deliberate slope along a gutter toward its outlets. Too little and water stands; too much and it overshoots. Hanger spacing is what keeps it over the years, and sagging changes it after installation.
Drip edge
Edge metal at eaves and rakes that carries water off the deck and into the trough rather than back behind it. Code-required, cheap at installation, and impossible to retrofit without lifting the covering.
Questions this calculator answers
What size gutter do I need?
Work out the flow first. Multiply horizontally projected roof area by rainfall intensity by 0.0104, then compare against IPC Table 1106.6. Size on its own answers nothing.
What does horizontally projected roof area mean?
The footprint the roof covers when viewed from above. Rain falls vertically, so a steep roof catches exactly as much water as the shadow it casts at noon. Using sloped area inflates the answer by the pitch multiplier.
What is the capacity of a standard 2×3 downspout?
IPC Table 1106.3 does not rate one. It rates a 2×2 at 30 gpm and a 2×4 at 92. Anyone quoting a single gpm for a 2×3 has invented it. This tool returns a bracket.
Does gutter slope change capacity?
Substantially. Table 1106.6 rates the same 1 1/2 × 2 1/2 profile at 26 gpm at 1/4 in/ft and 40 gpm at 1/2 in/ft, 54% more flow for double the fall. No quote you receive will state the slope it intends to hang.
Do these capacities apply to a K-style gutter?
Approximately, and only by nominal size. The table lists semicircles and rectangles. A residential K profile is neither.
Sources and methodology
Figures dated 9 August 2026. Last reviewed .
IPC 2018 §1106.4, Vertical walls: the sidewall contribution to drained area (International Code Council, retrieved 2026-09-05. codes.iccsafe.org returned HTTP 403 to direct automated retrieval of the section page on that date. The provision was read through the ICC public search surface and is recorded that way; Table 1106.6 is not reproduced.)
NOAA Atlas 14 / Precipitation Frequency Data Server (NOAA National Weather Service, Hydrometeorological Design Studies Center, retrieved 2026-09-05. The federal source for point rainfall intensity by duration and average recurrence interval. hdsc.nws.noaa.gov was unreachable to automated retrieval on that date. The programme page was read instead and no figures are restated here.)
29 CFR 1926.501(b), duty to have fall protection (OSHA, read at the Cornell LII mirror, retrieved 2026-09-05. osha.gov returned HTTP 403 to automated retrieval on 5 September 2026, so the text was read at the Cornell LII mirror and is cited that way.)
IPC Section 1106, Tables 1106.3 and 1106.6 (ICC via UpCodes, retrieved 2026-08-09. Every gpm figure on this page comes from these two tables. Section 1106.1 sets the design basis as the 100-year hourly rainfall rate.)