Gutter Calculator
Find how many feet of gutter to buy, whether you need 5-inch or 6-inch K-style, and how many downspouts, hangers, end caps, and outlets go with it.
Gutter runs
Enter each separate gutter run and the footprint (plan) area of roof that drains into it. Leave unused runs at 0.
Roof and rainfall
Roof area is the flat footprint, not the sloped surface — the pitch factor accounts for slope. Rainfall intensity is your area's design storm in inches per hour (see the rainfall section below).
Results
80 ft of 5" K-style gutter with 2 2x3 downspouts, 42 hangers, and 4 end caps.
Recommended gutter size
5" K-style
A 5" K-style gutter handles every run, with about 40% capacity to spare on the busiest one.
Five-inch is the standard residential size, so parts and colors will be easy to find.
Gutter length: 80 linear ft
Downspouts / outlets: 2 / 2
Hangers: 42
End caps (straight runs): 4
Roof pitch factor: 1.1
Run 1 adjusted area: 3,300 sq ft → 5", 1 downspout
Run 2 adjusted area: 3,300 sq ft → 5", 1 downspout
How this works
Gutter sizing uses the method from the SMACNA Architectural Sheet Metal Manual as simplified in residential sizing guides. For each gutter run, the calculator takes the flat (footprint) roof area draining into it, multiplies by a roof pitch factor (1.0 up to 3/12, 1.05 for 4–5/12, 1.1 for 6–8/12, 1.2 for 9–11/12, 1.3 for 12/12 and steeper) and by your maximum rainfall intensity in inches per hour. That adjusted area is compared with a 5-inch K-style gutter's rated 5,520 sq ft and a 6-inch's 7,960 sq ft; the smallest size that handles every run is recommended. Downspouts per run are the footprint area divided by 600 sq ft (2x3) or 1,200 sq ft (3x4), rounded up, with at least two outlets on any run 60 ft or longer. Hangers are the run length divided by your spacing plus one per run, and each straight run gets two end caps. Source: This Old House, How To Size Gutters and Downspouts. Rainfall intensity for your address: NOAA Precipitation Frequency Data Server.
Roof pitch factors
| Roof pitch | Pitch factor |
|---|---|
| Flat to 3/12 | 1.00 |
| 4/12 – 5/12 | 1.05 |
| 6/12 – 8/12 | 1.10 |
| 9/12 – 11/12 | 1.20 |
| 12/12 and steeper | 1.30 |
Gutter and downspout capacities
Gutter capacities are compared with adjusted area (footprint × pitch factor × rainfall in/hr). Downspout capacities are in plain footprint area, as the source states them.
| Component | Rated capacity |
|---|---|
| 5" K-style gutter | 5,520 sq ft adjusted area |
| 6" K-style gutter | 7,960 sq ft adjusted area |
| 2x3 in rectangular downspout | 600 sq ft of roof |
| 3x4 in rectangular downspout | 1,200 sq ft of roof |
Worked example: a gable ranch house
A single-story ranch is 48 ft long and 30 ft deep with a 6/12 gable roof and 1 ft overhangs all around. Gutters go on the two long eaves: each eave is 48 + 2 = 50 ft. Each side drains half the footprint: 50 ft × (30 + 2) ÷ 2 = 800 sq ft. The pitch factor for 6/12 is 1.1. With a design rainfall of 5 in/hr, the adjusted area per run is 800 × 1.1 × 5 = 4,400 sq ft — under the 5,520 sq ft rating, so 5-inch K-style works with about 20% to spare. Downspouts: 800 ÷ 600 = 1.33, so 2 per run with 2x3 downspouts, 4 in total, which also means 4 outlets. Hangers at 24 in: 50 × 12 ÷ 24 = 25, plus 1 = 26 per run, 52 total. End caps: 2 per run, 4 total. Total gutter: 100 linear ft.
A second example: a steeper roof in a stormier climate
Same 800 sq ft per side, but the roof is 10/12 and the local design rainfall is 7 in/hr. The pitch factor jumps to 1.2, so the adjusted area is 800 × 1.2 × 7 = 6,720 sq ft — over the 5-inch rating of 5,520 but under the 6-inch rating of 7,960. The recommendation becomes 6-inch K-style. Switching to 3x4 downspouts (1,200 sq ft each) means one per run by area: 800 ÷ 1,200 rounds up to 1. The runs are only 50 ft, so the 60-ft two-outlet rule doesn't apply — though with a 6-inch system many installers still add a second downspout at the far end to keep water from traveling the full 50 ft.
Common mistakes
Using the sloped roof area and the pitch factor together
The pitch factor already converts footprint to an effective catchment. If you take roof area from a shingle estimate (which is measured along the slope) and then multiply by the pitch factor too, you oversize every gutter. Enter the flat footprint here.
Sizing from the whole roof instead of each run
A gutter only carries the water from the roof that drains into it. A 2,400 sq ft gable house has roughly half on each side — so each gutter run sees about 1,200 sq ft, not 2,400. Enter each run with its own area; the calculator sizes them separately and recommends the largest size any run needs.
Leaving the rainfall at an example value
Rainfall intensity is the biggest multiplier in the formula — doubling it doubles the adjusted area. A value that suits a dry region will undersize gutters in a region with intense summer storms. Look up your own design intensity rather than using a placeholder.
Counting only the area rule for downspouts
A 70 ft run draining 500 sq ft needs only one 2x3 downspout by area, but water traveling 70 ft to a single outlet overflows in a heavy storm. Runs of 60 ft or more need at least two outlets regardless of area.
Forgetting where upper roofs dump onto lower ones
When a second-story downspout empties onto a lower roof, all that water ends up in the lower gutter. Add the upper roof's area to the lower run it drains onto — otherwise the lower gutter is sized for half the water it actually gets.
Pairing 6-inch gutters with 2x3 downspouts
A 6-inch gutter moves more water than a 2x3 downspout drains comfortably, and 3x4 outlets are the usual match. If the calculator recommends 6-inch, switch the downspout size to 3x4 and recheck the count.
When this calculator's model stops applying
- Valleys: two roof planes meeting in a valley concentrate water at one point of the gutter. Put an outlet near the valley or upsize that run; the area math alone doesn't capture that concentration.
- Very steep or very tall walls: a tall wall above a roof plane sheds wind-driven rain onto the roof below. SMACNA methods add part of the wall area to the drainage area; this calculator doesn't.
- Heavy snow and ice country: gutters may need closer hangers, heavier-gauge material, or heat cable, and ice dams can make any gutter overflow regardless of size.
- Corners and miters: runs are treated as straight lines. Inside and outside corners, and the extra hangers installers put near them, are counted separately.
- Round downspouts, half-round gutters, box gutters, and commercial systems use different capacity figures than the K-style and rectangular-downspout values used here.
- Underground drainage: downspout count assumes water is discharged at grade. Tying downspouts into buried pipe, a dry well, or a storm sewer needs pipe sized separately under your local plumbing code.
FAQ
How many feet of gutter do I need?
Add up the length of every eave that sheds water — the roof edges running along the bottom of each slope. A simple gable house has two eaves, each as long as the house plus any overhang at the gable ends; a hip roof has gutters on all four sides. Rake (sloped gable) edges don't get gutters. Buy the total plus a little for miters and splices if you're using sectional gutter; seamless gutter is formed on site to the exact length.
Should I use 5-inch or 6-inch gutters?
Five-inch K-style is the standard residential size and handles up to 5,520 sq ft of adjusted drainage area; six-inch handles about 7,960 sq ft. Adjusted area is the roof footprint draining to that run × the roof pitch factor × your rainfall intensity in inches per hour. If any run's adjusted area is over 5,520, go to 6-inch — and most people then use 6-inch on the whole house so the parts match.
How many downspouts do I need?
Divide the roof area draining to each gutter run by what one downspout can carry — about 600 sq ft for a 2x3 in rectangular downspout and 1,200 sq ft for a 3x4 — and round up. Any run 60 ft or longer should also get at least two outlets, even if the area math says one. The calculator applies both rules to each run and adds them up.
What is the roof pitch factor and why does it matter?
Rain rarely falls straight down; wind drives it into the roof, and a steep roof presents more surface to that wind-driven rain than its flat footprint suggests. Sizing guides multiply the footprint area by 1.0 for flat to 3/12, 1.05 for 4/12–5/12, 1.1 for 6/12–8/12, 1.2 for 9/12–11/12, and 1.3 for 12/12 and steeper. A steeper roof therefore needs more gutter capacity for the same footprint.
Where do I find my rainfall intensity?
Use the maximum short-duration rainfall intensity for your location, in inches per hour. NOAA's Precipitation Frequency Data Server (Atlas 14) publishes intensities by duration and return period for any US address; gutter sizing guides typically use a short, intense storm (a few minutes) rather than a daily total. A local gutter contractor or building department can also tell you the value they design to. The calculator's default of 5 in/hr is only an example starting point — replace it with your own figure.
Do I measure roof area along the slope or flat?
Flat — the footprint, as if you were looking down from above. Measure the horizontal depth from the eave to the ridge (or to wherever the roof stops draining toward that gutter) and multiply by the eave length. The pitch factor then accounts for slope. Measuring along the slope and also applying the pitch factor would count the slope twice.
How many gutter hangers do I need?
Hangers are usually spaced every 24 in along the gutter, and closer — 12 to 18 in — where heavy snow, ice, or tree debris loads the gutter. The calculator divides each run's length by your spacing and adds one so there's a hanger at each end. Check your hanger manufacturer's spacing requirement, since some hidden hangers are rated for wider spacing.
How many end caps and outlets do I need?
Each straight gutter run has two open ends, so it needs two end caps. Each downspout needs one outlet (the drop that connects gutter to downspout). Where a run wraps a corner, the corner is joined with an inside or outside miter instead of end caps; the calculator treats runs as straight and leaves miters for you to count, since they depend on your roof's shape.
Does one big roof section need a bigger gutter, or more downspouts?
Downspouts help carry water out of the gutter, but a gutter still has to move the water along to them. If a run is over a 6-inch gutter's rating, the usual fix is to split it: put an outlet near each end (or in the middle) so water flows in two directions and each half drains a smaller area.
How far apart should downspouts be?
There's no single code distance for houses, but the area and length rules above usually place downspouts at the corners of each run. On a run that needs two, putting one at each end and pitching the gutter toward both halves the distance water travels. Where the run needs more, space them evenly and slope each section toward its outlet.
What slope should gutters have?
Gutters are hung with a slight fall toward each outlet so water doesn't pond. A commonly cited figure is about 1/4 in of drop per 10 ft of run; check your gutter maker's installation guide. On long runs, a center-high layout draining to both ends keeps the fall from pulling the far end too far below the fascia.
How do I measure the roof area that drains to each gutter?
For each gutter run, measure the eave length and the horizontal distance from the eave to the ridge — or to the high point of whatever roof plane slopes down to that gutter. Multiply the two to get the footprint area. On a gable house that is roughly half the building footprint per side, including overhangs. On a hip roof, the triangular hip ends drain to their own short gutters, so give each end its own run. You can measure from the ground with a tape and your house plans, or from a satellite map's area tool, without climbing onto the roof.
Should I buy seamless or sectional gutters?
Either works with this calculator's numbers. Seamless aluminum gutters are roll-formed on site by an installer to the exact length of each run, so there are no joints to leak except at corners and outlets. Sectional gutters come in fixed lengths (often 10 ft) that you join with connectors, which suits DIY installs but adds seams. If you're buying sectional gutter, divide each run by the section length and round up, and add a connector for each joint.
Do gutter guards change the size I need?
Guards don't change the size calculation, but they can reduce how much water actually gets into the gutter in a downpour if they clog or if water sheets over them. In heavy-rain regions, pairing guards with a gutter sized at the upper end of what the calculator recommends gives some margin.
Does this calculator size box gutters or commercial systems?
No. It's built for residential K-style gutters using the capacities published in consumer sizing guides based on SMACNA methods. Box gutters, built-in gutters, half-round profiles, and commercial roof drainage are sized from the full SMACNA manual or the plumbing code's storm drainage tables, usually by a designer.
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