Gutter IQ
The short answer
A 6 inch gutter has about 44 percent more cross sectional area and moves about 54 percent more water than a 5 inch. Those are different numbers for a reason: a deeper channel has a better hydraulic radius, so flow rises faster than area does.
But the decision is usually not made in the trough. It is made at the outlet. Most residential overflow is downspout limited, so if a 5 inch gutter with 2 x 3 downspouts is overflowing, moving to 3 x 4 downspouts doubles the outlet capacity and is often the cheaper and more effective fix than upsizing the gutter.
On this page
- What the capacity numbers actually are
- Why nobody publishes a K-style cross section
- Rainfall intensity is the real decision variable
- Downspout pairing, and the cheaper fix
- Coil, gauge and material
- Cost, and why sources differ by 5x
- Fascia fit, and the myth about it
- When 6 inch is right and when it is overkill
- Half round
- Common questions
- Sources
What the capacity numbers actually are
Half round is the case where the geometry is exact and everyone can check the arithmetic, so start there. A 5 inch semicircular section is 9.8 square inches and a 6 inch is 14.1. That is 44 percent more area.
Now the code table, which is stated as the maximum horizontally projected roof area a gutter serves at 1 inch per hour of rainfall:
| Diameter | 1/16 in per ft | 1/8 in per ft | 1/4 in per ft | 1/2 in per ft |
|---|---|---|---|---|
| 4 in | 720 | 1,020 | 1,440 | 2,040 |
| 5 in | 1,250 | 1,770 | 2,500 | 3,540 |
| 6 in | 1,920 | 2,770 | 3,900 | 5,520 |
| 8 in | 4,640 | 6,560 | 9,280 | 13,120 |
The 5 inch to 6 inch step is 54 to 56 percent at every slope, consistently larger than the 44 percent area gain. That is not an error in the table. Open channel flow follows Manning’s equation, where flow depends on area and on hydraulic radius raised to the two thirds power. A deeper channel wets proportionally less perimeter for the water it carries, so it moves water faster as well as more of it.
Why nobody publishes a K-style cross section
We went looking for a published cross sectional area for K-style gutter from a manufacturer, a code body or a trade association, and could not find one anywhere. Manufacturers publish outside dimensions only. The code capacity table above covers semicircular gutters and does not cover K-style at all.
So be careful with K-style capacity figures you find online, including the widely copied pair of 5,520 and 7,960 square feet. Those trace back to a single table whose half round values line up with a level slope column while its K-style values line up with a sloped one, which means the K-style to half round comparison inside it is not like for like.
What we can say with confidence is the direction and the rough magnitude: a 5 inch K-style holds meaningfully more than a 5 inch half round, and a 6 inch K-style holds meaningfully more than a 5 inch K-style, with the half round figures above as the best sourced anchor available.
Rainfall intensity is the real decision variable
Every capacity figure above is quoted at 1 inch per hour. Nobody lives at 1 inch per hour. To use the table you divide by your local design rainfall rate, and that division does more to decide 5 versus 6 inch than roof area, pitch and profile combined.
A 5 inch semicircular gutter at a quarter inch per foot serves 2,500 square feet at 1 inch per hour. In a region with a 4 inch per hour design rainfall, the same gutter serves 625 square feet. That is why roof area rules of thumb quoted without a location are close to meaningless.
Two further things worth knowing, and they push in the same direction:
- Design storms differ between standards. Plumbing codes size on a 100 year, 60 minute storm. SMACNA’s sheet metal manual uses a 5 minute storm, which for the same city can be roughly twice the intensity. A gutter responds in minutes, not in an hour, so the shorter duration is the physically relevant one. This is the single best explanation for why field rules of thumb look so conservative next to code tables.
- Code rainfall data is old. The rainfall figures in the plumbing code have been essentially unchanged since 1995. The building enclosure institute IIBEC recommends comparing the code value for your location against current NOAA data, and documents cases where the gap alone is enough to move a specification from 5 inch to 6 inch.
Look up your own number on NOAA’s Precipitation Frequency Data Server. It takes about a minute and it is the input that matters most.
Downspout pairing, and the cheaper fix
| Gutter | Standard downspout | Upgrade |
|---|---|---|
| 5 in K-style | 2 x 3 (6.00 sq in) | 3 x 4 with an oversized outlet |
| 6 in K-style | 3 x 4 (12.00 sq in) | 4 x 5 on high volume work |
A 3 x 4 downspout has exactly double the cross section of a 2 x 3. By the published leader capacity tables the practical gap is larger still, roughly threefold, because vertical leader capacity scales with the dimension rather than with the area.
That matters for the decision on this page. If an existing 5 inch system overflows, the trough is often not the constraint. The outlet and the downspout usually are. Upsizing to 3 x 4 on the existing 5 inch gutter, with an oversized outlet to suit, is cheaper than replacing the whole run and frequently solves the actual problem. It is also the more honest recommendation to make to a homeowner who has been told they need 6 inch gutters.
Coil, gauge and material
5 inch K-style runs on 11 3/4 in coil, 6 inch on 15 in. That is 28 percent more aluminum per linear foot at the same gauge, which is the real material cost difference and the honest floor for any upcharge. Six inch is also more commonly specified in .032 rather than .027, which adds about another 20 percent of metal on top.
Full details and the yield table are on the coil calculator page.
Cost, and why sources differ by 5x
| Source | Quoted upcharge for 6 in |
|---|---|
| Angi, August 2026 | $3 to $5 per linear ft |
| LeafFilter, updated June 2026 | $0.50 to $1.00 per linear ft |
| GutterAll, supplier | $0.50 to $1.00 per linear ft |
Both ends are defensible once you see what each is counting. The 50 cent to a dollar figure is close to pure metal, and it lines up with the 28 percent coil difference. The three to five dollar figure is a whole specified upgrade: bigger downspouts, often a heavier gauge, more hangers, and margin on a change the customer requested. Neither is wrong, and quoting only one of them to a homeowner is how a bid gets argued about.
Worth knowing that at least some contractors charge the same for either size and treat it as a positioning decision rather than a cost pass through, which tells you the upcharge is more market driven than material driven.
Fascia fit, and the myth about it
You will read that 6 inch gutter needs a 1x8 fascia. Dimensionally that is not true.
| Item | Actual dimension |
|---|---|
| Nominal 1x6 fascia board | 5.5 in |
| 6 in K-style gutter, back height | about 4.75 in |
| 5 in K-style gutter, back height | about 3.5 in |
A 6 inch gutter mounts on a 1x6 with about three quarters of an inch to spare. The real objection is proportion: installers commonly say 6 inch looks better against a 1x8 or 1x10, and on small scale trim a 6 inch gutter can read heavy. That is a judgment about appearance, and it is worth telling a homeowner that is what it is rather than presenting it as a technical requirement.
The constraints that are real: hidden hangers are sized per gutter and 5 inch and 6 inch hangers are not interchangeable, and on a low pitch roof the taller front face of a 6 inch gutter is worth checking against the shingle overhang and drip edge geometry.
When 6 inch is right and when it is overkill
Go to 6 inch when:
- The drainage math says so once you have divided by your local rainfall intensity. This is the only reason that is not a judgment call.
- Valleys concentrate a large tributary area into a few feet of gutter. Whole roof area math misses this entirely, and it is a common cause of overflow on a system that is nominally sized correctly.
- The roof is metal, slate or tile. Slick surfaces shed faster and overshoot more.
- Downspout placement is constrained, so runs have to be long between drops.
- Gutter covers are installed or planned, since covers reduce effective intake.
- The roof is steep and large, where the pitch factor and the area compound.
It is probably overkill when:
- The house is an average suburban roof in a moderate rainfall region and the existing 5 inch system has never overflowed.
- The complaint is overflow at one corner, which is a drop placement or outlet size problem, not a trough size problem.
- The only argument for it is that bigger is better. It is more metal, more weight on the fascia, and a heavier look, for capacity the roof will not use.
Half round
Half round is sold in 5, 6, 7 and 8 inch, and both routes exist: it is run seamless on machines set up for it, generally in 5 and 6 inch only, and it is also bought pre formed in sections, which is the more common route above 6 inch. The reverse bead variant is the seamless friendly one, because the bead sits inside and allows hidden hangers.
On capacity, use the one size up rule: a 6 inch half round is roughly the equal of a 5 inch K-style. It is also the historically correct profile on pre 1950s, Colonial, Tudor and Victorian work, where a K-style ogee reads wrong. And it self scours better, since a curved section gives debris no corner to lodge in.
One underappreciated point: 6 inch reverse bead half round runs on 11 7/8 in coil against 15 in for 6 inch K-style, so it uses about 21 percent less metal per foot. That partly offsets its higher installed price.
Common questions
Is a 6 inch gutter worth it over a 5 inch?
It is worth it when the drainage math says so, and not otherwise. Step up to 6 inch for large or steep roof areas, high local rainfall intensity, rooflines with valleys dumping into a short section of gutter, metal or tile roofs that shed fast, and runs where you cannot place enough downspouts. On an average suburban house in a moderate rainfall region, 5 inch with correctly sized and correctly placed downspouts is not undersized, and upsizing it fixes nothing.
How much more water does a 6 inch gutter hold than a 5 inch?
Roughly 40 to 50 percent more cross sectional area, and roughly 50 to 55 percent more flow. Those are two different measures and the difference matters. For half round, where the geometry is exact, a 5 inch section is 9.8 square inches and a 6 inch is 14.1, which is 44 percent more area. The published roof area a 6 inch semicircular gutter serves is 54 to 56 percent more than a 5 inch at every slope, because a deeper channel has a better hydraulic radius and so moves more water than its area alone suggests.
What size downspout goes with a 6 inch gutter?
3 x 4 is the normal pairing, with 4 x 5 used on high volume commercial work. A 5 inch gutter normally takes 2 x 3, and moving that same 5 inch gutter to a 3 x 4 downspout is a common and often better value upgrade, because a 3 x 4 has exactly double the cross section of a 2 x 3. Most residential overflow is limited by the outlet and the downspout rather than by the trough.
Does a 6 inch gutter fit a 1x6 fascia?
Yes. A nominal 1x6 board is 5.5 inches actually, and a 6 inch K-style gutter has a back height of about 4.75 inches, so it mounts with room to spare. The common objection to 6 inch on 1x6 fascia is proportion, not fit: installers usually say 6 inch looks better against a 1x8 or 1x10. That is an aesthetic judgment, not a dimensional constraint, and it is worth being clear with a homeowner about which one you are making.
How much more does 6 inch gutter cost than 5 inch?
Published figures range from about 50 cents to a dollar per linear foot up to three to five dollars per linear foot, and both are defensible because they are measuring different things. The pure metal difference is small: 6 inch takes 15 inch coil against 11 3/4 inch, so about 28 percent more aluminum per foot. The larger figure reflects a whole specified upgrade, which usually also means 3 x 4 downspouts, often a heavier .032 gauge, more hangers, and margin on a job the customer has asked to upgrade.
Is a 6 inch half round the same as a 6 inch K-style?
No, and half round is the smaller of the two. A 6 inch half round is 14.1 square inches, while a 5 inch K-style is somewhere in the region of 11 to 13. As a working rule, step up one nominal size in half round to match a K-style: a 6 inch half round is roughly the equal of a 5 inch K-style. Half round does self scour better, because a curved section has no corners for debris to lodge in.
What size gutter do I need for my roof?
Take the roof area draining to that run, multiply by the pitch factor, and divide the published roof area figure for the gutter by your local rainfall intensity in inches per hour. Published capacity tables are stated at 1 inch per hour, so a gutter listed at 2,500 square feet serves about 625 square feet in a region with a 4 inch per hour design rainfall. Look your own rate up on NOAA’s Precipitation Frequency Data Server rather than using a national average, because the spread across the country is several fold.
Sources
Every figure on this page traces to one of these. Where two of them disagree, the page says so rather than averaging them.
- International Plumbing Code 2021, Table 1106.6, size of semicircular roof gutters (code document)
- Berger Building Products, Proper Gutter and Downspout Sizing (reprints SMACNA tables, cross sections and flow) (manufacturer, reprinting a trade association standard)
- IIBEC, Rainfall Intensity Changes Over Time: Have the Codes Kept Pace? (professional institute)
- NOAA Precipitation Frequency Data Server (US government)
- Gutter Supply, coil widths by profile (supplier)
- Angi, seamless gutter cost, updated August 2026 (consumer cost guide)
- This Old House, how to size gutters and downspouts (pitch factors, downspout rule of thumb) (edited trade press)