Gutter Sizes: 5-Inch vs 6-Inch, and How to Choose

Five-inch K-style is the standard residential gutter in the United States, and six-inch is the usual step up. Which one your house needs is not a matter of taste or of square footage alone — it is set by how much water arrives in the hardest five minutes of rain your area gets, which depends on the roof area feeding each run, how steep that roof is, and where you live. The same house needs a different gutter in Seattle than in New Orleans, by a factor of four.

The sizes you will actually be offered

K-style — the profile with the flat back and the moulded front face — comes in 5-inch and 6-inch as standard, with 7-inch available and rare on houses. Half-round comes in a wider set of diameters, commonly 5 and 6 inch on homes. Downspouts pair with them: 2 × 3 inch is the usual partner for 5-inch gutter and 3 × 4 inch for 6-inch, and that pairing matters as much as the trough — see gutter downspouts for the spacing and sizing rule, which is a separate decision from this one.

Going from 5-inch to 6-inch buys you more than the extra inch suggests.

That is 44% more cross-section, our arithmetic on those two figures. The capacity gain is larger still — about 54% at the same slope, on the table further down — because a deeper channel moves water more efficiently as well as holding more of it.

What actually sets the size

Three inputs, and only one of them is about your house’s size.

1. The roof area feeding each run

Not the whole roof. Each gutter run only has to carry the roof plane that drains into it, so a house with four runs is asking each one to handle roughly a quarter of the job. Measure the plan area of the roof plane above the gutter — its footprint on the ground, not its sloped surface.

2. The pitch of that roof

Rain does not always fall vertically, so a steep roof catches more than its footprint suggests. The sheet-metal trade’s standard handles this with a multiplier on the plan area:

Roof pitch	Multiply plan area by
Level to 3-in-12	1.00
4-in-12 to 5-in-12	1.05
6-in-12 to 8-in-12	1.10
9-in-12 to 11-in-12	1.20
12-in-12	1.30

Berger Building Products, Table 1-1, July 2007, reproducing the SMACNA Architectural Sheet Metal Manual.

Note what this table does not say. It does not tell you to use the roof’s true sloped area, and the source is explicit about why: “Experience has taught that use of the true area of a pitched roof often leads to oversizing of gutters, downspouts, and drains.” A steep roof catches more, but nowhere near as much more as its surface area implies.

3. How hard it rains where you live

This is the input every consumer gutter-size chart leaves out, and it has the widest spread of anything on this page.

What matters is intensity, not annual total, and those are close to unrelated. Seattle is famous for rain and sits near the bottom of this table, because its rain arrives as drizzle over many days. New Orleans is four times worse in the five minutes that decide whether your gutter overflows.

One caveat, stated plainly: that rainfall table is built on US Weather Bureau records through 1978 and was published in 2007. It is the oldest data on this site by a wide margin, and NOAA has since revised US rainfall frequency statistics. Use it for the shape of the problem and the relative difference between cities; check your own local design figure before sizing anything you are about to pay for.

Turning that into a number

The conversion between a wet roof and a flow rate is fixed arithmetic:

Worked through, and this is our arithmetic on their figures, not something any source states for your house:

Take a roof plane 40 feet along the gutter and 15 feet up the slope in plan — 600 square feet — on a 6-in-12 roof.

1. Design area: 600 × 1.10 = 660 square feet
2. In Seattle: 660 × 2.1 ÷ 96.15 = 14.4 gallons per minute
3. In New Orleans: 660 × 8.3 ÷ 96.15 = 57.0 gallons per minute

Same house, same roof, four times the flow. That is the whole argument for why a national “5-inch is fine up to X square feet” rule is not worth much.

Where the published tables stop, and where they disagree

Here the honest answer gets more complicated than most pages admit, in two separate ways.

The two standards differ by about a factor of two

Both the sheet-metal trade’s manual and the model plumbing code publish capacity for a semicircular gutter, in gallons per minute, at a stated slope. For the same gutter at the same slope they do not agree.

The code figure is almost exactly double in both cases. Across the five diameters the two tables share, the ratio runs between roughly 1.9 and 2.2.

We are not going to average them, and we are not going to tell you one is wrong. What the documents themselves say is that the sheet-metal manual’s charts come from National Institute of Standards and Technology experiments on level gutters, and that manual warns in its own text that “all gutters are not effective for their full depth and width” — which is design margin. The code section, as reproduced, states no basis at all. Our reading is that these are probably two different quantities rather than two answers to one question, but neither document says so, so we are not presenting that as fact.

What to do with that: size to the smaller figure. Under-sizing is the failure you are trying to avoid, and the cost of one size up is small against the cost of water going over the front edge into the soil at the wall.

Run the worked example against both and you can see why it matters. Our New Orleans case needed 57.0 gpm. On the sheet-metal figures a 5-inch is well short and a 6-inch lands right on it. On the code figures a 5-inch is comfortable. Same house, opposite answers.

Every one of those figures is for a half-round gutter

This is the gap that matters most, and it is the reason this page has no calculator on it.

Both tables above are for semicircular gutter. Almost every US house has K-style, and a 5-inch K-style is not a 5-inch semicircle — it is deeper and squarer, and it carries more. How much more, we cannot tell you.

The standard does not duck the question. It gives a method:

What it does not give is the K-style cross-sectional area that method needs, and neither does the plumbing code, which tabulates semicircles and rectangles only. Searching turns up consumer calculator sites quoting K-style capacities with no stated basis and no agreement with each other, which is not a source.

So the direction is knowable and the number is not: if your K-style gutter is close to the half-round limit for its nominal size, you are probably still inside its real capacity — but treat that as a reason not to panic, not as permission to under-size.

Why there is no calculator here

Every input a roof-sizing calculator needs is verified above except one, and the missing one is K-style’s capacity. We looked, twice, and we are not going to publish one. That is a decision, not an unfinished job.

What searching returns is K-style roof-area figures in quantity, none of them citing a manufacturer or a standard. One of them can be shown to be borrowed. A manufacturer’s own K-style page — a source we trust enough to use elsewhere on this site for hanger spacing — states that a 5-inch K-style covers up to 2,500 square feet of roof, and states on the same page that K-style holds 20% more water than the same size of half-round.

Both cannot be true there. 2,500 square feet is exactly the figure the sheet-metal manual publishes for a 5-inch half-round, printed further up this page. If K-style really carried 20% more, the number would be 3,000. What that page has done is take the half-round figure and put a K-style label on it.

So the circulating K-style capacities look like restatements of a figure for a different profile. Building a calculator on them would hand people a confident number off the wrong shape — and the error runs toward telling homeowners to buy larger gutter than they need, which being conservative does not make honest.

It stays withheld unless a manufacturer or a standards body publishes a K-style cross-sectional area, or a K-style capacity table that says where its numbers come from. Until then this page gives you the arithmetic and the half-round figures, tells you which way K-style differs, and stops there.

Level gutters, and the slope question

Every capacity figure above assumes a slope. A gutter hung dead level carries less:

Those are roof areas at one inch of rain an hour, so divide by your local intensity to get the real figure. On our 660-square-foot example, the 5-inch level gutter runs out at 2,500 ÷ 660 = 3.8 inches an hour — our arithmetic. Comfortable in Seattle at 2.1. Well short in New Orleans at 8.3.

Gutters are usually hung with a slight fall toward the downspout for exactly this reason, and a gutter that has lost its pitch over time is a capacity problem as well as a standing-water problem. That is a maintenance item — see gutter maintenance.

Common questions

Is 6-inch gutter worth the extra cost?

If your roof area per run is large, your roof is steep, or you are anywhere with intense summer storms, yes. The material premium is modest against the cost of a foundation problem, and the bigger trough also pairs with a 3 × 4 downspout that clogs far less readily than a 2 × 3. If you have a small, low-pitched roof in a low-intensity region and your existing 5-inch has never overflowed, there is no case for changing it.

My gutters overflow. Does that mean they are too small?

Not necessarily, and this is worth checking before you spend anything. Overflow at the outlets with a clear trough usually means too few or too small downspouts, not an undersized gutter. Overflow along the whole length in heavy rain is more likely a genuine capacity or pitch problem. Overflow at the back edge, running behind the gutter, is usually a flashing issue — see gutter apron.

Does a bigger gutter mean fewer downspouts?

No. The two are sized separately and the downspout is usually the binding constraint. A larger trough that still drains through the same outlets fills and overflows in the same storms. Size both.

What size gutter for a 2,000 square foot house?

There is no answer to that question as asked, which is why every page that gives one is guessing. A 2,000 square foot house might drain through four gutter runs or eight, have a 4-in-12 or a 12-in-12 roof, and sit anywhere from 1.7 to 17.4 inches an hour of design rainfall. Work it out per run, with your own pitch and your own local intensity.

Can I mix sizes on one house?

Yes, and it is sometimes the right answer. A long run under a large roof plane can be 6-inch while a short run over a porch stays 5-inch. It has to look deliberate rather than accidental, and the downspouts have to match each run.

How we check our figures

The pitch factors, the rainfall intensities and the half-round capacities are transcribed from a manufacturer’s technical reference that reproduces the SMACNA Architectural Sheet Metal Manual tables, read directly from the document rather than from a summary of it. The plumbing code figures were read from two independent reproductions of two different code editions and agree exactly; the code publisher’s own library blocks automated reading, so that check stands in for a primary read rather than replacing it.

Every arithmetic step on this page — the 44% area gain, the flow figures for the worked example, the 3.8 inches an hour — is ours, performed on those published figures and shown so you can redo it. Where a figure is not sourced, this page says so instead of estimating. See how we calculate costs.

Related reading

For the decision that pairs with this one, see gutter downspouts. For whether to choose K-style or half-round in the first place, see K-style vs half-round gutters. For what a new gutter run costs once you know the size, see how much gutters cost per foot and the gutter installation cost calculator.