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Gutter size calculator

Enter the area draining to one run and your local rainfall intensity. Then pick a gutter and downspouts from the plumbing code tables and see if they carry it.

0.0104 gpm per sq ft per in/hr A unit conversion, not a survey. One inch per hour on one square foot is 1/12 cu ft an hour, and NIST SP 811 puts a cubic foot at 7.4805 US gallons. (1/12 × 7.4805) ÷ 60 = 0.0104.

Flow in gallons per minute = projected roof area × rainfall intensity × 0.0104. For example, 2,000 sq ft at 3 in/hr sheds 62.4 gpm.

A 6-inch semicircular gutter at 1/8 in per foot is rated 110 gpm, and two 3-inch round downspouts at 92 gpm each carry it. Use your own intensity from NOAA Atlas 14.

Flow first, then the profile

One run at a time. Nothing is stored or emailed.

An example. The footprint, not the sloped surface. Add half of any wall that sheds onto this roof (IPC 1106.4).

Look up your location in NOAA Atlas 14. The code designs to the 100-year hourly rate.

— Flow this roof produces
— Gutter capacity
— Downspout capacity
— Does this combination carry it?

What this assumed —

IPC Chapter 11 sizes building storm drainage; a K-style gutter matches its table by nominal size only. A check, not a design. HyreRoof does not install gutters.

Gutters are sized by water, not by house

A gutter has to carry the water that reaches it in the worst minutes of a storm. That depends on two numbers: the projected roof area draining to that run, and your local rainfall intensity.

Intensity is regional. A place with many wet days can still have lower peak rates than a place with fewer, heavier storms, so look up your own rather than borrowing someone else’s gutter size.

HyreRoof analysis: the calculator checks two limits, the gutter and the downspouts. Either one can be the one that fails.

What this calculator does not model

  • Your local rainfall intensity

    You type it. The calculator does not look it up. Use NOAA Atlas 14 for your location.

  • Where the downspouts go

    It sizes capacity. On a long run, outlet spacing can be the real limit.

  • Where the water goes afterward

    Dumping a sized system at the base of a wall moves the problem to the foundation.

  • Snow and ice loads

    In cold climates gutters also carry weight, and hanger spacing matters.

The area draining into your gutter can be larger than your roof

On a house with an upper story, a dormer wall or a taller building next door, the plumbing code adds part of that wall to the area.

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.”

Wind-driven rain strikes the wall and runs down onto the roof below, so the code counts half of that wall as roof.

Two adjacent walls of equal height add 35 percent of their total area. Unequal adjacent walls add 35 percent of the common height plus 50 percent of the rest of the taller wall. Two opposite walls of equal height add nothing, since only one can face the wind at a time.

How this was read. codes.iccsafe.org returned HTTP 403 to direct automated retrieval of this section page on September 5, 2026. The provision and its wall-configuration cases were read through the ICC public search surface and are recorded that way. The gutter capacities are the Table 1106.6 ratings listed in the calculator above.

This is model code. Adoption and amendment are local, and your jurisdiction may have adopted another edition. Confirm with your building department before relying on it.

A worked example

A two-story house with a one-story rear extension. The extension roof is 300 sq ft. The main-house wall above it is another 300. The code says to size for 300 + 150 = 450 sq ft, half as much water again as the roof alone.

Attached garages, dormer cheeks, chimneys and a neighbor’s wall on the lot line do the same on a smaller scale. For a run like this, type the roof area plus the wall share into the calculator.

How to get your own rainfall intensity

Intensity moves the answer more than any other input, and it is free to look up for your exact location.

Drainage sizing uses a rainfall rate in inches per hour, for a stated duration and recurrence interval. The federal source is NOAA Atlas 14, published through the National Weather Service’s Precipitation Frequency Data Server by latitude and longitude.

Duration matters. A gutter holds a few minutes of water. A short cloudburst can fill it faster than a long, steady rain that delivers more water in total.

The recurrence interval is a choice. IPC 1106.1 designs building storm drainage to the 100-year hourly rate. A lower design storm accepts more frequent overflow. What sits under the gutter, a lawn or a doorway, helps you decide.

What we do not do. We do not restate NOAA’s figure for your location. It is a lookup, so the calculator starts the intensity at zero and waits for yours. Where your jurisdiction has adopted design values, those govern.

Why a gutter that is big enough can still overflow

A gutter is a system: the roof edge, the trough, the outlets and where the water goes. The trough is only one part.

PartHow it failsWhat that means
Outlet count and spacingToo few outlets on a long run, or all at one end. Water has to travel the length of the trough to leave.The far end can overtop while the outlet still has spare capacity.
Outlet conditionLeaves and grit plug the drop, the narrowest point in the system.The whole run is limited to what the open outlets can take. A bigger trough does not help.
Fall along the troughToo little and water stands. Too much and it runs past the outlet. Sagging hangers change it over time.Standing water can freeze in winter and hold debris year-round.
Back edge and drip edgeWater tracks back 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.”Water behind the gutter wets the fascia and soffit, out of sight until the paint fails.
Discharge pointA sized system dumps concentrated water at the base of a wall.The gutter problem becomes a foundation problem. Extensions, splash blocks or a drain line are their own design.
Guards and screensDebris bridges the mesh, or heavy rain sheets over a solid cover.A guard keeps debris out. It does not add capacity, so check the size first.
Secondary drainage on enclosed roofsIf the main drain blocks, water has nowhere to go. “secondary emergency overflow roof drains or scuppers shall be provided where the roof perimeter construction extends above the roof”On a roof with a parapet, trapped water is a load. The code requires the second path.

HyreRoof analysis, built on the code text 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 quote should say

A gutter quote can be a price per foot and a color. Ask for these too.

  • The drained area, with any wall share shown

    Projected roof area plus the sidewall addition, with the arithmetic.

  • The rainfall intensity used

    A number, a duration and a recurrence interval, from NOAA Atlas 14 or your jurisdiction’s design values.

  • Outlet count and spacing

    Where each downspout goes and how far apart they are, not just how many.

  • The fall, and the hanger spacing that holds it

    A stated fall toward each outlet, with hangers close enough to keep it. Ask about spacing in snow country.

  • Where the water goes after the downspout

    Splash block, extension, buried line or a drain connection.

  • Gutters hung dead level

    A gutter with no fall holds water. It can look neat against the fascia and still stand full.

  • Sizing from the house footprint

    Each run drains its own part of the roof. Size each run on what flows into it.

  • A guard offered as the fix for overflow

    A guard does not add capacity. Ask what caused the overflow first.

Gutter work is work at height

Gutter work does not feel like roof work, but the eave is the same height.

The federal rule. 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.”

What that means for you. OSHA rules bind employers, not homeowners, but the line still applies. A trained roofer may not work above six feet without fall protection. Look from the ground, a window or the attic, or use a photo or a drone.

The practical version. Clean from the ground with a scoop on a pole or a hose attachment, or hire it out. If you use a ladder, keep both feet on it, move it instead of reaching, and never step from it onto the roof.

Gutter and drainage terms

Horizontally projected area
The roof plane seen from directly above, not its sloped surface. Drainage sizing uses this, not the slope factor used for materials.
Sidewall contribution
The share of an adjoining wall the plumbing code adds to the drained area: half of one wall, 35 percent of two adjacent equal walls, nothing for two opposite equal walls.
Rainfall intensity
Inches of rain per hour for a stated duration and recurrence interval. NOAA Atlas 14 publishes it by location.
Recurrence interval
The 10-, 25- or 100-year label on a design storm. A probability: a 100-year storm has about a 1% chance in any year.
Outlet or drop
The opening from the trough into the downspout. The narrowest point, and the first to block.
Leader or conductor
The plumbing code’s words for the downspout.
Fall
The slope along a gutter toward its outlets.
Drip edge
Edge metal at eaves and rakes that carries water off the deck and into the gutter.

Embed this calculator

You can put this gutter and downspout sizing calculator on your own site, free. Paste the code below into your page’s HTML. Keep the credit link under it. The tool runs in the reader’s browser and stores nothing.

Questions this calculator answers

What size gutter do I need?
Work out the flow first: projected roof area × rainfall intensity × 0.0104. Then compare it with the gutter ratings in IPC Table 1106.6. Size on its own answers nothing.
What does horizontally projected roof area mean?
The area the roof covers seen from above. For drainage sizing, rain is treated as falling straight down, so a steep roof catches as much as its shadow at noon. Sloped area overstates it.
What is the capacity of a 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. This tool gives a 2×3 a range between those two rather than one number.
Does gutter slope change capacity?
Yes. Table 1106.6 rates the same 1 1/2 × 2 1/2 profile at 26 gpm at 1/4 in per foot and 40 gpm at 1/2 in per foot: 54% more flow for twice the fall. Ask what fall the installer will use.
Do these ratings apply to a K-style gutter?
Only roughly, by nominal size. The table lists semicircular and rectangular gutters. A K profile is neither, so treat the match as approximate.
Where do I find my rainfall intensity?
NOAA Atlas 14, on the National Weather Service’s Precipitation Frequency Data Server. Enter your location and read the rate in inches per hour for the duration and recurrence interval you are designing to.

Sources and methodology

Figures dated September 5, 2026. Last reviewed .

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