Gutter Calculator
Calculate how many gutter sections, downspouts, hangers, end caps and elbows you need from the total eave length, with an optional material cost per linear foot.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Gutter sections to buy
- 12 sections
- 10 ft sections for 120 ft of gutter, plus 8 seam connectors
- Downspouts
- 4 downspouts
- About one every 35 ft; each needs about 10 ft of downspout per story plus 2 straps
- Hangers
- 64 hangers
- One every 24 in plus one extra per run for the ends
- End caps
- 8 end caps
- 2 per run; where two runs meet at a corner, use an inside or outside corner piece instead of 2 end caps
- Elbows
- 12 elbows
- 3 per downspout: 2 to offset from the fascia to the wall, 1 at the bottom to kick water away
Gutter formula
Sections = Total eave length / section length, rounded up
Downspouts = Total eave length / downspout spacing, rounded up
Hangers = Total eave length (in) / hanger spacing + 1 per run
End caps = 2 x runs; Elbows = 3 x downspoutsWorked example
A house with 180 ft of eave in 6 runs, using 10 ft aluminum sections, downspouts every 40 ft and hangers every 24 in: 180 / 10 = 18 sections and 18 - 6 = 12 seam connectors. Downspouts: 180 / 40 = 4.5, so 5. Hangers: 180 x 12 / 24 = 90, plus 6 for the run ends = 96. End caps: 2 x 6 = 12. Elbows: 3 x 5 = 15.
At $8 per linear ft for the gutter itself, 180 ft costs $1,440 before downspouts, hangers and fittings.
Quick reference
| Eave length | Sections | Downspouts | Hangers | Elbows |
|---|---|---|---|---|
| 60 ft | 6 | 2 | 30 | 6 |
| 80 ft | 8 | 3 | 40 | 9 |
| 100 ft | 10 | 3 | 50 | 9 |
| 120 ft | 12 | 4 | 60 | 12 |
| 150 ft | 15 | 5 | 75 | 15 |
| 200 ft | 20 | 6 | 100 | 18 |
| 250 ft | 25 | 8 | 125 | 24 |
How to measure for gutters
Measure each eave that needs a gutter along the fascia and add them up. Count each straight length as a run; a run that turns a corner becomes two runs joined by a corner piece. Divide the total by the section length, 10 or 20 ft, and round up. Sections join with slip connectors or seamers, one for each joint, so the number of connectors is sections minus runs. A 20 ft section halves the joints, and every joint is a future leak, so it is worth the trip to a supply house.
Seamless gutters are roll-formed from .027 or .032 aluminum coil on a truck-mounted machine, so the truck has to park within reach of each eave. Seams remain only at miters and end caps. Pitch is 1/16 to 1/8 in per ft, so a 40 ft run pitched one way drops 2-1/2 to 5 in and looks crooked against the fascia. The installer sets a high point in the middle and puts a downspout at each end instead.
Downspouts, hangers and fittings
Plan a downspout for every 30 to 40 ft of gutter, and one at each end of any run over 40 ft. A 2x3 downspout drains about 600 sq ft of roof; a 3x4 drains 1,200 and passes the leaves that plug a 2x3. Each downspout needs about 10 ft of pipe per story, two elbows at the top to return to the wall, one at the bottom, and two straps per story. A splash block moves water about 3 ft. An extension or a buried 4 in pipe to daylight or a drywell gets it 6 ft or more from the foundation.
Hidden hangers with 2 in screws into the rafter tails every 24 in are standard, 18 in in snow country; spike-and-ferrule hangers work loose. Check the fascia for rot and the drip edge for a gutter apron before hanging anything, because the gutter hides both. Every run gets two end caps, or a miter where two runs meet at a corner, and every joint, cap and outlet gets tripolymer gutter sealant, not silicone. Many towns ban downspouts into the sanitary sewer and some require a drywell or storm connection, so ask before you set the bottom elbows.
What this calculator leaves out
Downspout pipe, outlets, straps, miters, splash blocks or extensions, sealant (one 10 oz tube per 6 to 8 joints) and touch-up paint. Also fascia repair, gutter apron, leaf guards, hauling off the old gutters, and any drain pipe or drywell. A seamless quote covers most of this per ft installed, so it is not comparable to the price per ft of sections.
Mistakes that cost money
- Running gutters dead level to look straight: pitch 1/16 to 1/8 in per ft or standing water rots the fascia and breeds mosquitoes.
- Spike-and-ferrule hangers into bare fascia work loose in a few winters; use hidden hangers with 2 in screws into the rafter tails.
- One 2x3 downspout on a 50 ft run overflows at the far end; a 2x3 drains about 600 sq ft, a 3x4 about 1,200.
- Downspouts dumped at the foundation: a splash block gets water 3 ft out at best; run an extension or buried pipe 6 ft or more.
- Hanging new gutters on rotted fascia or over missing drip edge: replace the board first and add a gutter apron so water cannot run behind.
- Sealing sectional joints with silicone: it peels off aluminum; use a tripolymer gutter sealant and lap seams in the direction of flow.
Key facts
- 5 in K-style is the most common residential gutter; 6 in is used for large or steep roofs and heavy rainfall regions.
- Sectional gutters come in 10 ft lengths at big box stores and 20 ft from supply houses. Seamless gutters are roll-formed on site from coil stock in one piece per run, up to 40 ft or more.
- A 2 x 3 in downspout drains about 600 sq ft of roof and a 3 x 4 in downspout about 1,200 sq ft, which is why the rule of thumb is one downspout per 30 to 40 ft of gutter.
- Gutters slope toward the downspout 1/16 to 1/8 in per ft, about 1 in over a 16 ft run; any steeper shows against the fascia.
- Hangers go every 24 in, or every 18 in where snow and ice load the gutter.
Frequently asked questions
How many downspouts do I need?
One for every 30 to 40 ft of gutter, and at least one per run. 120 ft of gutter in four runs needs about 4 downspouts.
Are gutter guards worth it?
Micro-mesh guards screwed to the gutter lip keep out shingle grit and pine needles and are what most installers put on now. Reverse-curve guards overshoot in a hard rain and void the shingle warranty if screwed through the roof. Guards add weight, so tighten hangers to 16 in, and they still need a yearly rinse. Branded systems typically cost more per ft than the gutter itself.
Are seamless gutters worth it?
Sectional gutters have a joint every 10 ft and each one leaks within 5 to 10 years. Seamless is .027 or .032 aluminum, thicker than box-store sections, hung in one visit with seams only at corners. For any run over 20 ft it is the better value over the life of the roof.
What size gutter do I need?
5 in K-style with 2x3 downspouts handles most homes. Use 6 in with 3x4 downspouts when a single run drains more than about 5,000 sq ft of roof, on roofs steeper than 8/12 that shed water fast, or in the Gulf Coast and Southeast rain belt.
Sources and references
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Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.