Whether you need 5-inch or 6-inch gutters comes down to three numbers on your house: the spacing of the downspouts, the steepness of the roof, and the amount of roof feeding each run. Building researchers at the University of Illinois say the smaller gutter suffices only when a run passes all three of their tests. Most other roofs are better off with 6-inch.
Below, the tests are applied to three sample roofs, alongside NOAA rainfall figures, downspout sizing and the cost gap.
How do 5-inch and 6-inch gutters differ?
In width, and in the metal it takes to form them. Most house gutters come 5 or 6 inches wide, according to the Illinois notes. Englert’s catalog shows the 5-inch K-style starting from an 11-3/4-inch strip of coil and the 6-inch from a 15-inch strip, so the bigger trough uses more metal.
We found no primary source that gives a verified capacity ratio between the two, so be wary of any precise percentage. The sizing test below is more useful anyway.
Either width can be bent into the K-style (ogee) profile, which the Illinois guide ranks as the most popular shape for both appearance and capacity. Half-rounds hold less water for their width, a point that matters on older homes; our K-style and half-round comparison covers the trade-offs.
| Feature | 5-inch K-style | 6-inch K-style |
|---|---|---|
| Coil strip it is formed from (Englert) | 11-3/4 inches | 15 inches |
| When the Illinois guide says it is enough | Downspouts closer than 30 feet, pitch no steeper than 9:12, roof area below 30 x run length | Any run that fails a test |
| Extra cost (HomeGuide, national) | Starting point | Roughly $1 to $2 more a linear foot |
| Where HomeGuide says it fits | The typical house | Big roofs or heavy rain |
How do you know if a 5-inch gutter is big enough?
Test each gutter run separately. The 5-inch size only works if the run clears all three bars:
- Downspout spacing: the downspouts sit closer than 30 feet apart along the run.
- Roof pitch: 9:12 or flatter, so the roof climbs no more than 9 inches for each foot.
- Roof area: the true surface area draining into the run comes in under 30 times the run’s length in feet.
Fail one test and the guide’s answer is 6-inch. The notes date from 1990, so treat them as a rule of thumb from a university building research council, not a code requirement.
Measure the real roof, not the footprint
The guide wants actual roof area rather than plan area, since a steeper roof needs a bigger gutter: a steeper plane spreads more surface over the same footprint. To convert, multiply the plan area by the factor for its pitch.
| Roof pitch | Multiply plan area by |
|---|---|
| 4:12 | 1.05 |
| 6:12 | 1.12 |
| 8:12 | 1.20 |
| 9:12 | 1.25 |
| 10:12 | 1.30 |
| 12:12 | 1.41 |
Plan area is the length of the run times the level distance from the gutter back to the ridge, or to wherever that roof plane ends. Include any higher roof whose water spills onto the lower one, since it all reaches the same gutter. The factors are plain geometry, and HomeGuide’s roof guide applies the same idea when it turns a 2,000-square-foot one-story house into roughly 2,235 square feet of roof at a 6:12 pitch.
What do the tests look like on a real house?
The numbers below are examples, not measurements of any particular home.
Example 1: a ranch with a long front run
A 40-foot front gutter drains one plane that runs 14 feet back to the ridge at 6:12. Actual area is 40 x 14 x 1.12, about 627 square feet. The area limit is 40 x 30, or 1,200, so that test passes, and 6:12 passes the pitch test.
With one downspout at each end, though, the downspouts sit 40 feet apart and fail the spacing test. A third downspout near the middle brings the spacing to about 20 feet and lets a 5-inch gutter work. If that middle downspout would land in front of a door or a picture window, a 6-inch gutter is the cleaner answer.
Example 2: a steep 10:12 roof
A 26-foot run sits under a 10:12 plane that is 12 feet deep. Actual area is 26 x 12 x 1.30, about 406 square feet, well under the 780-square-foot limit (26 x 30). Downspouts at each end are 26 feet apart, which passes. The pitch fails, so this run calls for 6-inch even though it drains a modest area.
Example 3: an addition under a taller roof
A 27-foot gutter on a one-story family room catches its own 12-foot-deep roof at 4:12, about 340 square feet (27 x 12 x 1.05). It also catches the main roof above, 16 feet deep at 8:12, which drains down onto the addition: about 518 square feet (27 x 16 x 1.20).
The total, about 859 square feet, tops the 810-square-foot limit (27 x 30). This run fails on area even though its spacing and pitch pass, so it needs a 6-inch gutter.
How heavy does rain get in central New Jersey?
Heavy enough in short bursts to matter. NOAA’s rainfall frequency server (Atlas 14) estimates that New Brunswick’s 10-year, 5-minute rate is about 6.3 inches an hour, against about 2.0 inches an hour for the 10-year, 60-minute rate. Put another way, a storm of 10-year strength has roughly a one-in-ten chance of arriving in any year.
| Storm (New Brunswick) | Peak 5 minutes | Full hour |
|---|---|---|
| 2-year | 4.76 inches an hour | 1.38 inches an hour |
| 10-year | 6.30 inches an hour | 2.00 inches an hour |
| 25-year | 7.08 inches an hour | 2.35 inches an hour |
| 100-year | 8.18 inches an hour | 2.89 inches an hour |
The 5-minute rates run about three times the hourly ones, which shows how hard rain falls in a storm’s peak minutes. Other towns in our area are close: about 6.7 and 2.1 inches an hour (10-year, 5- and 60-minute) at Toms River, and 6.3 and 1.9 at Newton.
The state climatologist’s office adds that most of New Jersey sees 25 to 30 thunderstorms a year.
Which storm a gutter should be sized for depends on the design method, and methods differ. Treat the Illinois tests as the decision rule, and the NOAA numbers as the reason a borderline run deserves the bigger gutter.
How many downspouts do you need, and what size?
Size downspouts by roof area: one square inch of outlet area for every hundred square feet of roof. Under that common rule, cited in the Illinois guide, a 2×3-inch downspout with 6 square inches of opening handles roughly 600 square feet. A 3×4-inch, with 12, handles roughly 1,200.
Where the downspouts go counts too. The guide wants one at every major corner, plus one in the middle when corners sit more than 50 feet apart or the run bends. Amerimax’s installation instructions call for bigger or additional downspouts on wide roofs, in downpour-prone areas and wherever gutters overflow.
Back to the examples. The ranch’s 627 square feet needs about 6.3 square inches of outlet, so one 2×3 is just short and two or three cover it. The addition’s 859 square feet needs about 8.6, which means two 2x3s or one 3×4.
When is a 6-inch gutter the better call?
When any Illinois test fails and extra downspouts cannot fix it, or when the roof piles water into one spot. Common cases:
- Roofs steeper than 9:12, which fail the pitch test outright.
- Runs that collect water from an upper roof or a dormer above.
- Long runs where a middle downspout would land in front of a door, a window or a walkway.
- Valleys that empty into the gutter, where two roof planes deliver their water to one point.
- Large, simple roof planes whose area tops the run length times 30.
- Houses getting gutter guards. This Old House notes reverse-curve guards are weaker in heavy water flow, so the gutter behind a guard is worth sizing with margin.
On smaller, simpler roofs that clear all three tests, 5-inch remains the sensible pick.
How much more do 6-inch gutters cost?
Not much per foot. HomeGuide’s 2026 national cost guide prices the step up to 6-inch at roughly $1 to $2 more a linear foot. Over the 100 to 200 feet of gutter it says most homes have, that comes to about $100 to $400.
For context, HomeGuide’s installed range is $12 to $25 a linear foot, which it puts at $2,200 to $5,000 for a typical house, and each downspout adds $30 to $75. Angi splits the cost out: $4 to $9 a foot for seamless aluminum material and $4 to $10 a foot for labor. These numbers are national, not New Jersey quotes, and your exact price comes from a free measurement.
With Form ST-8, new gutters escape New Jersey sales tax as a capital improvement; repairs do not, per the Division of Taxation.
What else matters besides size?
How the gutter is hung. The Illinois guide asks for aluminum hangers spaced 32 inches or closer, tightening to 16 to 24 inches where snow and ice linger, and an expansion joint on any run over 40 feet. Amerimax’s instructions pitch the gutter down roughly 1/4 inch every 10 feet toward the downspout.
The fascia has to be sound first, and HomeGuide advises fixing rotted fascia before new gutters go up. Our soffit and fascia page covers fascia repair, our seamless gutter page covers coil thickness and hanging, and our gutter guard page covers what goes on top. Every option we offer is on the gutters page.
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Gutter size questions
Is a 6-inch gutter always the better choice?
No. It is a bigger trough, but it only earns its keep where the roof needs it. The University of Illinois tests are downspouts closer than 30 feet, a pitch no steeper than 9:12, and roof area below 30 times the run length. Pass all three and 5-inch works; otherwise choose 6-inch.
Does a steep roof need bigger gutters?
Usually 6-inch. The Illinois guide rules out 5-inch gutters above a 9:12 pitch. It also says to count the true roof surface rather than the footprint, since a steeper roof spreads more surface over the same footprint.
How many downspouts does a gutter need?
Plan on one square inch of outlet area for every hundred square feet of roof, with a downspout at each major corner and another between corners more than 50 feet apart. That works out to roughly 600 square feet per 2×3 and 1,200 per 3×4.
How much more do 6-inch gutters cost than 5-inch?
HomeGuide’s 2026 national guide puts the upgrade at roughly $1 to $2 more a linear foot, about $100 to $400 over the 100 to 200 feet most homes have. Those numbers are national; your exact price comes from a free measurement.
Can I switch from 5-inch to 6-inch gutters when I replace them?
Yes. Check the fascia first, since HomeGuide advises fixing rotted fascia before new gutters go up. Use the replacement to recheck downspout size and placement too.
Sources
- University of Illinois Small Homes Council-Building Research Council, 1990 council notes on gutters and downspouts (F13.1)
- NOAA National Weather Service, rainfall frequency estimates for New Jersey (Atlas 14, Volume 2)
- Englert, seamless gutter catalog
- Amerimax, installation sheet filed with the City of Detroit
- Rutgers, state climatologist overview
- HomeGuide price guides: gutters, roof replacement, soffits and fascia
- Angi, seamless gutter price article
- This Old House, gutter guard comparison
- NJ Division of Taxation, contractor tax bulletin S&U-3
Gutters sized to your roof
Find out which gutter size each run needs
We measure every run, its roof area and pitch, and list the gutter size, coil thickness and downspouts in a written estimate.
