Gutter Length & Downspout Calculator

Total the gutter you have to buy from your eave runs, and find how many downspouts carry it away — the fast first pass before you size the profile.

Working on a roof is dangerous — falls are a leading cause of construction deaths. Measure from the ground, from plans or from photos where possible, use proper fall protection if you must go up, and consider hiring a licensed roofing professional. Results are planning estimates, not a bid.

Calculator

LF
Add every roof edge (eave) that will carry a gutter.
ft
Rule of thumb: one downspout every 30 to 40 ft (use 35).
Gutter length120 LF
Downspouts4
Spacing ruleone per 35 LF

120 LF of eaves needs about 120 LF of gutter and 4 downspouts (one every 35 ft; put them at corners and low points).

Gutters run along the eaves — the low, horizontal roof edges — so the linear feet you buy is simply the sum of those runs. The rake edges (the sloped edges up a gable) do not get gutters, so leave them out. Measure each eave along the fascia, add them up, and you have the footage; most suppliers sell K-style in 10-ft sections or roll seamless gutter to length on site, so round up to whole feet.

Downspouts are what actually move the water off the roof and away from the foundation. The industry rule of thumb is one downspout for every 30 to 40 linear feet of gutter, which this tool applies at a default of 35 ft. That is a starting count: put outlets at corners and at the low end of every slope, and add one wherever a long single run would otherwise send all its water to one end. Big roofs, steep roofs and cloudburst climates need the outlets sized as well as counted — do that on the gutter size and downspout capacity tools.

Formula

Two lines of arithmetic:

gutter_LF = Σ eave_runs\ndownspouts = ceil( gutter_LF ÷ feet_per_downspout )

The count is rounded up — you cannot install a fractional outlet, and it is always safer to drain a little more than a little less.

Worked example

A simple rectangular house with 120 ft of eaves, at one downspout per 35 ft:

gutter_LF   = 120 ft\ndownspouts  = ceil( 120 ÷ 35 ) = ceil(3.43) = 4

So order about 120 ft of gutter and 4 downspouts, placing them at the four corners so no single run carries more than roughly 30 ft to an outlet.

Placing the downspouts

The count tells you how many outlets to buy; where they go matters just as much. Favor the corners of the house so downpipes tuck out of sight, and always drop an outlet at the low end of a sloped run. If a run is long enough to need two outlets, splitting it and pitching the gutter both ways from a high center keeps the water shallow at each end (see gutter slope). Finally, get the water away from the house: extensions or buried drains should discharge several feet from the foundation, because a correctly sized gutter that dumps at the wall has only moved the problem down a story.

Frequently asked questions

Do I put gutters on the rake edges too?
No. Gutters go on the eaves (the low horizontal edges). The rakes are the sloped edges running up a gable, and water sheds off the eave below them, so they carry no gutter. Only add up eave length here.
How many downspouts do I need for 120 feet of gutter?
At one outlet per 35 ft, 120 ÷ 35 = 3.43, which rounds up to 4 downspouts — typically one at each corner. Very steep roofs or heavy-rain regions may want the outlets sized on the downspout capacity tool rather than just counted.
Is one downspout every 40 feet enough?
Forty feet is the loose end of the 30-to-40-ft rule and works for modest roofs in average rainfall. For larger drainage areas or intense storms, tighten the spacing toward 30 ft or step up to larger 3×4 in outlets so water does not back up and overflow the front lip.
Should I measure gutter length along the roof or the ground?
Measure the eave itself, along the fascia board where the gutter hangs. Because the eave is horizontal, the roof pitch does not lengthen it — unlike a rafter, a gutter run is the plain horizontal edge length.
Does this tell me the gutter size?
No — length and downspout count are separate from profile size. To choose between 5-inch and 6-inch K-style, use the gutter size tool, which weighs the drainage area, roof pitch and your local rainfall intensity.

Related calculators

Gutter SizeDecide between 5-inch and 6-inch K-style gutters from the roof drainage area, the roof pitch factor and your local rainfall intensity in inches per hour.Downspout CapacityFind how many 2x3-inch or 3x4-inch downspouts a roof needs from its adjusted drainage area, using industry outlet capacities at one inch per hour.Gutter SlopeWork out how much to drop a gutter run toward the downspout at a quarter inch per ten feet, including long runs pitched from a high center both ways.Siding CostEstimate your total siding cost from the squares you are ordering and your own installed price per square. No price list is stored in the tool.