Gutter Calculator
Gutter sections & hangers for your roofline
Replacing gutters? Enter the total roofline length to estimate the sections and hangers you need. Below: a roofline chart, how to size 5 inch against 6 inch gutter by the roof area draining into it, how many downspouts, and the slope that decides whether any of it works.
Sections and hangers by roofline
Used as a gutter sections calculator, the counts below assume standard gutter hanger spacing of 2 ft.
Gutter comes in 10 ft sections at the store; seamless gutter is rolled and cut to length on site by a contractor, which is why it has no joints to leak. Hangers go about every 2 feet.
| Roofline | With 5% | 10 ft sections | Hangers | Downspouts |
|---|---|---|---|---|
| 40 ft | 42 ft | 5 | 21 | 1 to 2 |
| 60 ft | 63 ft | 7 | 32 | 2 |
| 80 ft | 84 ft | 9 | 42 | 2 to 3 |
| 120 ft | 126 ft | 13 | 63 | 3 to 4 |
| 160 ft | 168 ft | 17 | 84 | 4 to 5 |
| 200 ft | 210 ft | 21 | 105 | 5 to 7 |
Tighten hanger spacing to 18 inches in snow country. Ice loading is the thing that pulls gutters off a fascia, and the extra hangers cost very little against a spring repair.
Gutter size by roof area
The right gutter size is not decided by the length of the run but by how much roof drains into it. Undersized gutter overflows in exactly the downpours it most needs to handle.
| Gutter | Downspout | Roof area it handles | Typical use |
|---|---|---|---|
| 4 in K-style | 2 × 3 in | up to ~2,500 ft² | Sheds, porches |
| 5 in K-style | 2 × 3 in | up to ~5,500 ft² | Most homes |
| 6 in K-style | 3 × 4 in | up to ~8,000 ft² | Large or steep roofs |
| 7 in | 4 × 5 in | above 8,000 ft² | Commercial |
Three things push you up a size regardless of area: a steep pitch, which throws water into the gutter faster than a shallow one; a high local rainfall intensity, measured as inches per hour rather than annual total; and long unbroken runs. If you are already replacing everything, going from 5 to 6 inch adds maybe 15% to the material bill and roughly 40% more capacity — usually the best value decision on the job.
Work out the roof area draining into each run with the roofing calculator, remembering that it is the sloped area rather than the footprint that sheds the water.
How many downspouts
The working rule is one downspout per 30 to 40 feet of gutter, and one at minimum for every separate run. More is almost always better: doubling the downspouts on a long run halves how far water has to travel along the gutter, which cuts overflow at the far end and reduces the debris that settles in the middle.
- Put them at the ends of runs, not the middle, unless the run is long enough to warrant both.
- Avoid walkways and doorways. A downspout that discharges across a path makes ice in winter.
- Extend at least 3 to 4 ft from the foundation, or further on clay soil. Water dumped at the base of a wall is the leading cause of wet basements.
- 3 × 4 downspouts clog far less than 2 × 3, because most blockages are leaf clumps that pass through the larger section.
Slope is what makes it work
A gutter must fall toward its downspout by about ¼ inch per 10 feet. That is barely visible by eye and completely essential: a dead-level gutter holds standing water, which breeds mosquitoes, accelerates corrosion and freezes into a solid block in winter.
On a run longer than 40 ft, slope from the centre down toward a downspout at each end rather than trying to fall the whole way in one direction — otherwise the far end sits an inch and a half low, which looks wrong from the street.
String a line before you hang anything. Chasing the fascia board is a common mistake, because fascia is often not level itself, and a gutter that follows a sagging fascia will pond in the middle.
Materials and what else to order
| Material | Cost per ft | Life | Notes |
|---|---|---|---|
| Vinyl | $3 to $6 | 10 to 20 yr | DIY friendly, brittle in cold |
| Aluminium | $6 to $12 | 20 to 30 yr | The standard choice |
| Steel | $8 to $15 | 20 to 30 yr | Stronger, can rust |
| Copper | $25 to $40 | 50+ yr | Architectural |
Beyond the gutter and hangers, budget for end caps (two per run), inside and outside corners, outlets where each downspout drops, elbows (usually two or three per downspout to get back to the wall and then out), downspout brackets every 6 to 8 ft, and sealant for every joint. Sectional gutter leaks at joints before anything else fails, so seal them properly and expect to re-seal in a decade.
How this gutter calculator works out its numbers
Sections are roofline × (1 + waste) ÷ 10 ft, rounded up, and hangers are the same length divided by the 2 ft spacing, with a 5% default waste allowance.
- Waste is lower here than on most materials because gutter is cut end to end with little offcut. 5% covers corner cuts and a spare section.
- Downspout counts in the table apply the one-per-30-to-40-ft rule and are guidance, not a calculator output.
- Drainage capacities are approximate figures for K-style gutter at a standard slope. Real capacity depends on rainfall intensity in inches per hour, roof pitch and the number of downspouts, and a designer would size against a local rainfall table.
- Seamless gutter is rolled on site in continuous lengths, so the section count applies to sectional gutter bought by the piece. The linear footage is what a seamless installer will quote against.
- Not included: end caps, corners, outlets, elbows, downspout material and brackets, sealant and gutter guards.
- Cost ranges are US planning figures gathered September 2026.
Reviewed September 2026.
Figures reviewed . Every worked example on this page is checked against the calculator above.
Gutter Calculator: frequently asked questions
How much gutter do I need?
Measure the total roofline that needs draining, then add about 5%. A 120 ft roofline needs 126 ft, which is 13 ten-foot sections plus 63 hangers at 2 ft spacing.
How far apart should gutter hangers be?
About every 2 feet, tightening to 18 inches in regions with heavy snow. Ice loading is what pulls gutters off a fascia, and extra hangers are far cheaper than a spring repair.
What size gutters do I need?
5 inch K-style handles up to roughly 5,500 ft² of roof and suits most homes. Go to 6 inch with 3×4 downspouts above that, or for steep roofs and heavy-rain regions. Size by the roof area draining into the run, not by the run's length.
How many downspouts do I need?
One per 30 to 40 feet of gutter, and at least one on every separate run. More is generally better, since it shortens how far water travels along the gutter and cuts overflow at the far end.
How much slope should a gutter have?
About ¼ inch of fall per 10 feet toward the downspout. On runs over 40 ft, slope from the centre toward a downspout at each end so the far end does not finish visibly low.
Is 6 inch gutter worth it over 5 inch?
Usually yes if you are replacing everything anyway. It adds roughly 15% to the material cost and about 40% more capacity, and paired with 3×4 downspouts it clogs far less often.
What else do I need besides gutter and hangers?
End caps (two per run), inside and outside corners, an outlet at each downspout, two or three elbows per downspout, downspout brackets every 6 to 8 ft, and sealant for every joint on sectional gutter.
How far should a downspout extend from the house?
At least 3 to 4 feet, and further on clay soil that drains slowly. Water discharged at the base of a wall is the most common cause of a wet basement, and an extension costs a few dollars.