How much gutter do I need?

The length of the eaves, not the perimeter. A gable roof sheds off two sides only, so a 40 by 28 ft house with a 12 in overhang needs 84 ft of gutter against a 144 ft perimeter. A hip roof is the exception where the two agree, because every edge is the bottom of a slope.

How do you calculate roof water flow?

Roof area times rainfall intensity times 0.0104. That constant is a unit conversion: an inch per hour over a square foot is 1/12 of a cubic foot, which is 7.48052 gallons, over 60 minutes. A 1,409 sq ft roof at 4 in/hr sheds 58.6 gallons a minute.

How many downspouts do I need?

Divide the flow by the rated capacity of the downspout you are buying, then check that against any maximum spacing. Whichever asks for more governs. For a 58.6 gpm roof with 9 gpm downspouts that is seven, spaced about 12 ft apart, which is far more than the usual rules of thumb suggest.

Does the building code size gutters?

No. IRC Chapter 9 was read in full for this page and mentions gutters exactly once, in a flashing clause. Roof drainage sizing lives in the plumbing code and in SMACNA's sheet metal manual, neither of which is free to read, which is why this page computes flow and asks you for the product's capacity.

Should I buy bigger gutters or more downspouts?

More downspouts, almost always. The flow in a length of gutter is the roof draining to it, so adding a downspout in the middle of a run halves the peak flow with no change to the gutter. Going up a gutter size is a smaller increase and it changes the brackets, the outlets and the look of the house.

Does the roof overhang count?

Yes, and it matters here more than anywhere, because the overhang is the part directly above the gutter. Roof area for drainage is the sloped area including the overhang, which on a 40 by 28 house at 6:12 with a 12 in eave is 1,409 sq ft against a 1,120 sq ft footprint.

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Gutter Calculator

Flow from unit arithmetic, because the IRC does not size gutters

A gable roof sheds water off two sides, so its gutter is not its perimeter. And the IRC does not size gutters at all: Chapter 9 mentions them once, in a flashing clause.

Gutter and downspout takeoff

Building length

Building width

Pitch, rise per 12

Eave overhang

Roof shape

Rainfall intensity (NOAA Atlas 14, your location)

in/hr

Gutter capacity (off the product)

gpm

Downspout capacity (off the product)

gpm

Maximum downspout spacing (yours or the maker's)

Flow is unit arithmetic and needs no source. Capacities are yours, off the product, because the sizing tables live in the plumbing code and in SMACNA and neither is free to read.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

Which edges actually get gutter

Worked example

A 40 ft by 28 ft gable roof at 6:12 with a 12 in overhang, at 4 in per hour.

The gutter runs 84 feet, not 144. A gable roof sheds off the two long sides only; the gable ends shed nothing. A perimeter-based order buys 60 feet you will never hang, which is 71% over.
The roof area is 1,409 sq ft, sloped and including the overhang. The overhang matters here more than anywhere: it is the part directly above the gutter.
The flow is unit arithmetic, not a table. An inch of rain over a square foot in an hour is 0.0104 gallons a minute, so 1,409 sq ft at 4 in/hr is 58.6 gpm off the roof. That conversion is exact and needs no source.
Seven downspouts at 9 gpm each, which works out at 12 ft apart along the run. The spacing rule alone would have said three, so the flow rule governs here.
And the gutter is comfortable, because the downspouts are close. The worst segment carries 7.3 gpm against the 12 you entered. Adding downspouts is how you fix an overloaded gutter, not buying a bigger one.

The number that matters is the flow, and the number that fixes it is the downspout count. Gutter size is the third variable, and it is the one everybody argues about first.

The formula

The only constant on this page, and it is a unit conversion rather than a figure:

gpm = roof area × intensity × 0.0104

roof area: sloped and including the overhang, because that is what sheds into the gutter
intensity: inches per hour at your location, from NOAA Atlas 14, which is free
0.0104: one inch per hour over one square foot, in gallons per minute: 1/12 cu ft, times 7.48052 gallons, over 60 minutes

That constant is arithmetic, so it is the same everywhere and always will be. Everything else on this page is either your measurement or your product's rated capacity, because the tables that would turn a flow into a gutter size are not free to read.

The IRC does not size gutters, and that is checkable

Chapter 9 of the IRC is roof assemblies, and it mentions gutters once. The whole chapter was read for this page, from Seattle's adopted code, and the single occurrence is inside a flashing clause about directing water away from a sidewall. There is no sizing table, no downspout rule and no rainfall term.

Roof drainage sizing lives in the plumbing code and in SMACNA. The IPC has tables for it and SMACNA publishes the architectural sheet metal manual that the trade actually uses. Neither is free to read, so neither can be quoted here.

Which is why this page computes the flow and asks you for the capacity. The flow is unit arithmetic and cannot be wrong. The capacity of a K-style 5 inch gutter at a given slope is a manufacturer's figure, and it varies with the slope you hang it at, which is a decision made on site.

Rainfall intensity is local and free. NOAA Atlas 14 publishes precipitation frequency by location and duration, and a five minute duration at a ten year return period is the usual basis for residential gutters. A national average would be as wrong here as a national electricity price, and for the same reason.

What this page will not do is print a rule of thumb. One downspout per 600 square feet is the usual one, and it has no source, no rainfall term and no gutter size in it.

Prices for roof coverings and tear-off

The published figures for roof coverings and tear-off are on the roof replacement cost calculator, with the byline, the publication date and the caveats that go with them. They are not repeated here, because the same table on two pages is worth less than one table you can find.

This site publishes no price of its own. Why, and what that means for these figures.

Why the perimeter is the wrong number

Gutter is sold by the foot and estimated off a plan, and the plan has a perimeter on it, so that is what gets used.

Water only leaves a roof at the bottom of a slope. On a gable roof that is two edges. The gable ends are the top of a triangle: water runs along them, not off them, which is why they get a rake board rather than a gutter.

On this roof that is 84 feet against a 144 foot perimeter, so ordering by perimeter is 71 percent over. On a long narrow house it is worse, because the two eaves are a smaller share of the perimeter as the building gets narrower.

A hip roof is the exception, and the only case where the two numbers agree: every edge is the bottom of a slope, so every edge gets gutter.

And a shed roof is the other extreme, with one gutter carrying the whole roof. Same area, same rain, all of it arriving at one edge, which is why a shed dormer or a lean-to needs downspouts out of proportion to its size.

Adding a downspout beats buying a bigger gutter

When a gutter overflows the instinct is to fit a larger one. That is usually the more expensive way to fix it and often the less effective.

The flow in any length of gutter is the roof draining to that length. Put a downspout in the middle of a run and each half now carries half as much, because the water in each half has somewhere closer to go. Doubling the downspouts halves the peak flow in the gutter, with no change to the gutter at all.

Gutter capacity does not scale nearly as fast as that. Going from a 5 inch to a 6 inch K-style is a meaningful increase and it is not a doubling, and it changes the brackets, the end caps, the outlets and the look of the house.

Downspouts also fail more gracefully. A gutter at capacity overflows along its whole length, over the fascia and down the wall. A downspout at capacity backs up locally, which is visible and fixable.

The one caveat is where the water then goes. More downspouts means more discharge points, and each of them needs somewhere to drain that is not the foundation it is standing next to.

Frequently asked questions

How much gutter do I need?: The length of the eaves, not the perimeter. A gable roof sheds off two sides only, so a 40 by 28 ft house with a 12 in overhang needs 84 ft of gutter against a 144 ft perimeter. A hip roof is the exception where the two agree, because every edge is the bottom of a slope.
How do you calculate roof water flow?: Roof area times rainfall intensity times 0.0104. That constant is a unit conversion: an inch per hour over a square foot is 1/12 of a cubic foot, which is 7.48052 gallons, over 60 minutes. A 1,409 sq ft roof at 4 in/hr sheds 58.6 gallons a minute.
How many downspouts do I need?: Divide the flow by the rated capacity of the downspout you are buying, then check that against any maximum spacing. Whichever asks for more governs. For a 58.6 gpm roof with 9 gpm downspouts that is seven, spaced about 12 ft apart, which is far more than the usual rules of thumb suggest.
Does the building code size gutters?: No. IRC Chapter 9 was read in full for this page and mentions gutters exactly once, in a flashing clause. Roof drainage sizing lives in the plumbing code and in SMACNA's sheet metal manual, neither of which is free to read, which is why this page computes flow and asks you for the product's capacity.
What rainfall intensity should I use?: A local figure, not a national one. NOAA Atlas 14 publishes precipitation frequency free by location and duration, and a five minute duration at a ten year return period is the usual basis for residential gutters. Intensities vary by a factor of several across the country.
Should I buy bigger gutters or more downspouts?: More downspouts, almost always. The flow in a length of gutter is the roof draining to it, so adding a downspout in the middle of a run halves the peak flow with no change to the gutter. Going up a gutter size is a smaller increase and it changes the brackets, the outlets and the look of the house.
Does the roof overhang count?: Yes, and it matters here more than anywhere, because the overhang is the part directly above the gutter. Roof area for drainage is the sloped area including the overhang, which on a 40 by 28 house at 6:12 with a 12 in eave is 1,409 sq ft against a 1,120 sq ft footprint.

Check these numbers yourself

2018 Seattle Residential Code, Chapter 9 Roof AssembliesSeattle Department of Construction and Inspections. Read 2026-09-09. Read in full to confirm the claim on this page: the chapter mentions gutters once, inside a flashing clause, and contains no sizing table for them.
NOAA Atlas 14, Precipitation Frequency Data ServerUS National Weather Service. Free precipitation frequency estimates by location, duration and return period. This is where the rainfall intensity input should come from; it is local by construction and a national average would be meaningless.

How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-09. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Capacities are the manufacturer's, not this site's. Gutter and downspout capacity depend on profile, size and the slope the gutter is hung at, and the tables that publish them are in the plumbing code and in SMACNA, neither free to read. The flow calculation is unit arithmetic and is exact. Rainfall intensity is local and comes from NOAA. This page does not tell you where the water goes after the downspout, which is a drainage question and matters more than the gutter.