Gutter Calculator
Measure the eaves that get gutters, count the separate runs and enter the eave height. You'll get a full parts list for K-style gutters and the slope to set on each run.
- Tested formulas
- Feet or metric
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2 What to buy
Updates as you type| Gutter runs | 4, about 30 ft each |
|---|---|
| Slope | ¼" per 10 ft toward the downspout: 0.75" drop per run |
| End caps | 8 |
| Downspout outlets | 4 |
| Downspout pipe | 4 10 ft sections |
| Elbows | 12 (3 per downspout: 2 at the eave, 1 at the bottom) |
That's the minimum: one downspout per run, plus a common rule of thumb of at least one per 40 ft of gutter (from Amerimax's vinyl guide). Check each long run on its own, and add downspouts where a big or steep roof drains into one run.
Shopping list
Quick answer
Divide total gutter length by 10 for sections and add hangers every 24 inches. 120 ft of gutter in 4 runs takes 12 sections, 64 hangers, 8 end caps and 4 downspouts with 12 elbows. Slope each run ¼ inch per 10 ft toward its downspout.
What goes into a gutter job
Gutters come in 10 ft sections (or formed on site in one piece by an installer). Each run needs an end cap at each end, an outlet for its downspout, and hangers along its length. Downspouts are sold in 10 ft lengths, and each one usually takes three elbows: two to bring it back from the gutter to the wall and one to turn the water away at the bottom.
Worked example: 120 ft of gutter in 4 runs
- Gutter: 120 ÷ 10 = 12 sections.
- Hangers every 24 inches: 120 × 12 ÷ 24 = 60, plus one at the end of each run = 64.
- Downspouts: one per run, and at least one per 40 ft of gutter (Amerimax's vinyl guide, a fair rule for metal too), so 4. At a 10 ft eave that's 40 ft of pipe, 4 sections.
- End caps: 8. Elbows: 12.
- Each 30 ft run drops 0.75 inch toward its downspout at ¼ inch per 10 ft.
Slope and hanger spacing
- Slope: Amerimax's metal gutter guide calls for ¼ inch of fall per 10 ft, and Spectra's says 1 inch per 40 ft, the same thing. Amerimax's vinyl guide uses ⅛ inch per 10 ft.
- Hangers: every 24 inches for metal and vinyl. Amerimax moves metal hangers to every 18 inches where snow and ice load the gutters.
- Long runs: put the downspout in the middle and slope both halves toward it, or add a downspout at each end. That halves the drop on each side.
5 or 6 inch gutters?
5 inch and 6 inch K-style gutters are the common sizes for houses. The right one depends on how much roof drains into each run, how steep it is and how hard it rains where you live. Big or steep roofs and heavy-rain areas push toward 6 inch gutters with the larger 3 × 4 inch downspouts. A local supplier or installer can size it for your roof; the parts count above is the same either way.
Frequently asked questions
How many downspouts do I need?
At least one per run. Amerimax's vinyl guide also says at least one per 40 ft of gutter, a fair rule of thumb for metal gutters too. Add more where a big or steep roof drains into one run, or where water overflows in heavy rain.
How far apart should gutter hangers be?
Every 24 inches is standard. In heavy snow and ice areas, Amerimax recommends every 18 inches for metal gutters.
How much slope does a gutter need?
About ¼ inch per 10 ft for metal gutters (1 inch per 40 ft), per Amerimax and Spectra. Amerimax's vinyl guide uses ⅛ inch per 10 ft.
How long is a gutter section?
Store-bought gutter and downspout pieces come in 10 ft lengths. Installers can also form one-piece gutters on site, with no joints along the run.
Should I get 5 inch or 6 inch gutters?
Both are common on houses. Bigger or steeper roofs and areas with heavy downpours push toward 6 inch gutters with larger 3 × 4 inch downspouts. A local supplier can size them for your roof area and rainfall; the parts count here is the same either way.
Last reviewed 2026-10-23. See how we calculate for our sources and assumptions.