Do I need 5-inch or 6-inch gutters?

Compare your adjusted drainage area (plan area × pitch factor × rainfall in/hr) to the capacities: 5-inch handles about 5,520 sq ft, 6-inch about 7,960. Under 5,520 a 5-inch works; above it, step up to 6-inch.

What is rainfall intensity and where do I find it?

It is your local peak rainfall in inches per hour. Look it up from NOAA precipitation-frequency data for your ZIP code, then enter it in the gutter size calculator.

Does roof pitch affect gutter size?

Yes. Steeper roofs drive rain faster and catch more wind-driven rain, so a pitch factor from 1.0 (low slope) up to 1.30 (12/12+) multiplies the drainage area.

Are 6-inch gutters always better?

Not always necessary, but worth it for large or steep roofs, long runs, valleys concentrating flow, or heavy-rain regions. Six-inch gutters also use larger 3×4 downspouts that clog less with leaves and needles. If your property has heavy tree cover or a history of overflow, sizing up is cheap insurance even when the drainage math would allow the smaller profile.

What size gutters do I need? 5-inch vs 6-inch K-style

Undersized gutters overflow in heavy rain no matter how well they are hung. Sizing is a drainage calculation — roof area, pitch and your local rainfall — not a guess. Here is how to run it.

By Francesco Zinghinì

Why gutter size is a drainage problem

A gutter has to carry the water that lands on the roof draining into it. Two houses with the same eave length can need different gutters if one has a bigger, steeper roof in a rainier area. Sizing compares the adjusted drainage area a gutter must handle against the gutter's rated capacity. Get it right and the gutter runs full but does not overflow; get it wrong and heavy rain sheets over the front edge, dumping water at the foundation exactly where you don't want it.

The three inputs

Adjusted drainage area combines three things:

adjusted area = plan area × pitch factor × rainfall intensity

Plan area: the horizontal (footprint) area of the roof draining to that gutter run — not the sloped area. Water falls vertically, so drainage uses the plan projection.
Pitch factor: steeper roofs drive rain into gutters faster and catch more wind-driven rain, so a multiplier is applied — 1.0 up to 3/12, 1.05 for 4–5/12, 1.10 for 6–8/12, 1.20 for 9–11/12, and 1.30 for 12/12 and steeper.
Rainfall intensity: your local peak rainfall in inches per hour, which you look up from NOAA precipitation-frequency data for your ZIP. A cloudburst region needs bigger gutters than a dry one.

The capacities

K-style gutters at proper slope handle, at 1 in/hr of rain: 5-inch about 5,520 sq ft of adjusted area, 6-inch about 7,960 sq ft. These industry/SMACNA-derived figures are on the gutter capacity chart. Compare your adjusted area to them: at or under 5,520, a 5-inch works; between there and 7,960, step up to 6-inch; above that, use 6-inch plus extra downspouts to shed the volume.

Worked example

A roof plane drains 700 sq ft of plan area, at 6/12 pitch (factor 1.10), in an area with 1.5 in/hr peak rainfall. Adjusted area = 700 × 1.10 × 1.5 = 1,155 sq ft. That is well under 5,520, so 5-inch K-style is fine. Bump the rainfall to a stormy 3 in/hr and it becomes 2,310 sq ft — still under 5,520, still 5-inch. Large, steep roofs in heavy-rain regions are where 6-inch earns its keep. Run your own numbers in the gutter size calculator.

K-style vs half-round, seamless vs sectional

Most US homes use K-style (the profile with a flat back and a decorative front that looks like crown molding) because it holds more water than a half-round of the same nominal width and fits standard hangers. Half-round gutters, common on older or high-end homes, carry less for the same size, so lean toward the larger option if you choose them. Separately, seamless gutters (rolled on site from one continuous coil) leak far less than sectional gutters joined every 10 feet, though both use the same capacity math — the seams affect leaks and maintenance, not sizing.

When to choose 6-inch anyway

Even when the math says 5-inch, consider 6-inch if the roof has long runs feeding few downspouts, valleys dumping concentrated flow onto one gutter, lots of overhanging trees that shed debris, or a history of overflow. Six-inch gutters also pair with larger 3×4-inch downspouts that clog less with leaves and pine needles. The downspouts matter as much as the trough — size them with the downspout calculator and space them with the gutter length tool.

Gutters are only half the system

A correctly sized trough still fails if the outlets can't keep up, so size the gutter and the downspouts together. A 6-inch gutter feeding a single undersized 2×3 downspout will back up and overflow in a downpour despite its capacity, because water leaves only as fast as the outlets drain it. Match larger gutters to larger 3×4 downspouts and enough of them — work the count from the downspout calculator and the spacing from the gutter length tool. Maintenance is the other half: any gutter overflows once it clogs with leaves and grit, so the "right size" assumes it's kept clear. In heavy tree cover, size up and consider guards, and clean them at least seasonally. The capacity math tells you the trough won't be the bottleneck; keeping the outlets sized and the channel clear is what makes that capacity real in a storm. When you run the size calculation, sketch the downspout locations at the same time, so you can confirm each run has an outlet close enough to shed its share of the water.

Estimate, and stay safe

This is a planning estimate from standard capacities; your local rainfall and roof geometry are yours to supply, and local codes or an installer may recommend otherwise. Gutter work is at height — working on or near a roof is dangerous, so use proper fall protection or hire a professional installer.