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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

ft
ft
in
in
in/hr
gpm
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ft

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

This figure is drawn from the roof you type in above.
Which edges actually get gutter

Gutter prices

Per linear foot
ItemLowHighUnit
Gutters installed, all types$4$40per linear ft
Aluminum$3$15per linear ft
Vinyl$3$7per linear ft
Galvanized steel$5$20per linear ft
Copper$15$40per linear ft
Faux copper$5$10per linear ft
Zinc$15$30per linear ft
Removing old gutters$1$2per linear ft

Source: HomeAdvisor, how much does it cost to replace gutters. Last price check: September 13, 2026.

The source's headline is $629 to $1,735, $1,181 on average, which it says works out to $4 to $40 per linear foot. At $4 a foot $1,735 buys 434 ft of gutter; at $40 a foot $629 buys 16 ft. The per foot rates are used here.

Gutter guard prices

Per linear foot
ItemLowHighUnit
Gutter guards, installed$6$13per linear ft
LeafGuard, installed$15$45per linear ft
Master Shield, installed$18$21per linear ft
All American Gutter Protection, installed$15$20per linear ft
Micro-mesh guards, material$2$4per linear ft
Foam guards, material$2$3.50per linear ft
RainDrop, material$4$5per linear ft
EasyOn, material$3$4per linear ft

Source: HomeAdvisor, how much do gutter guards cost. Last price check: September 13, 2026.

The source's overview calls $653 to $2,459 a price per linear foot; it is a whole-job total. Its own rate is $6 to $13 a foot, and each of the three professional brands it names starts at $15.

Gutter cleaning prices

As each row states
ItemLowHighUnit
Gutter cleaning, typical job$119$234total
Gutter cleaning$0.95$2.25per linear ft
One story house$0.95$1.25per linear ft
Two story house$1$1.85per linear ft
Three story house$1.25$2.25per linear ft
Downspout flushing and minor repairs$50$100added

Source: HomeAdvisor, how much does gutter cleaning cost. Last price check: September 15, 2026.

At the source's $0.95 to $2.25 per foot, the 150 to 200 ft of gutter on many houses is $143 to $450, so the top of its typical range assumes a short run.

Downspout prices

As each row states
ItemLowHighUnit
Vinyl downspout, replaced$5$8per linear ft
Aluminum downspout, replaced$5$7per linear ft
Galvanized steel downspout, replaced$10$11per linear ft
Removing and disposing of old downspouts$100$100average per job
Downspout drainage system, where none exists$160$160average
Gutter repair, typical job$194$642total

Source: HomeAdvisor, how much does gutter repair cost. Last price check: September 16, 2026.

The source's table gives downspouts per linear foot, and its text gives the same $5 to $11 per installation. Its gutter replacement guide says a replacement price includes the downspouts.

Seamless gutter prices

As each row states
ItemLowHighUnit
Seamless gutters, typical project$658$1,797total
Seamless gutters$3$25per linear ft
Seamless aluminum$4$9per linear ft
Seamless vinyl$3$5per linear ft
Seamless galvanized steel$8$10per linear ft
Seamless copper$15$25per linear ft
Labor$4$10per linear ft
Other materials and supplies$3.50$3.50per linear ft
100 ft of seamless gutter$1,400$1,400average
Gutter guards$7$10per linear ft

Source: HomeAdvisor, how much do seamless gutters cost. Last price check: September 16, 2026.

The source's length table prices every size at exactly $14 per linear foot, which is above its own $4 to $9 for aluminum, the material it calls the most popular.

Worked example

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

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.

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.
How much do seamless aluminum gutters cost?
HomeAdvisor's June 2026 guide prices aluminum at $3 to $15 per linear foot and all gutter types at $4 to $40. For the 84 feet of gutter in the example that is $252 to $1,260 in aluminum. Seamless gutter is formed on site from a coil to the length of each run, so the eaves length above is the figure to measure it by.
How much do gutter guards and gutter covers cost?
HomeAdvisor's gutter guard guide gives $6 to $13 per linear foot installed, which is $504 to $1,092 for 84 feet. LeafGuard, one of the brands it names, is $15 to $45 per foot installed. Guards and covers are priced on the same gutter length, which is the eaves and not the perimeter.
How much does gutter cleaning cost?
HomeAdvisor puts a typical gutter cleaning at $119 to $234, or $0.95 to $2.25 per linear foot, rising with the number of stories: $0.95 to $1.25 a foot on one story and $1.25 to $2.25 on three. The gutter length this calculator gives is the number to multiply.
How much does it cost to install gutters and downspouts?
HomeAdvisor's June 2026 replacement guide gives $4 to $40 per linear foot of gutter installed and says that price includes the downspouts. Its repair guide prices a replaced downspout on its own at $5 to $8 per linear foot in vinyl, $5 to $7 in aluminum and $10 to $11 in galvanized steel, about $100 to remove and dispose of the old ones, and about $160 for a downspout drainage system where a house has none. Downspout screens are about $8 each and splash blocks about $10.
How much do seamless gutters cost?
HomeAdvisor's June 2026 guide gives $658 to $1,797 for a typical project and $3 to $25 per linear foot, with seamless aluminum at $4 to $9, vinyl $3 to $5 and copper $15 to $25. Labor is $4 to $10 per linear foot.
How much does it cost to replace gutters and fascia?
HomeAdvisor's June 2026 guide gives $100 to $1,100 for gutter replacement and $6 to $20 per linear foot for fascia in HomeAdvisor's June 2026 guide. New gutters alone are $4 to $40 per linear foot.
How much do gutter cleaning services cost?
HomeAdvisor's June 2026 guide gives $119 to $234 for a typical cleaning and $0.95 to $2.25 per linear foot, more on a taller house.

Check these numbers yourself

How every figure here is verified: Sources & Method. Who builds this: , and the method is on 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.

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More of them: all guides.

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.