The Definitive Guide to Gutter Sizing

gutter sizing

half round and k style gutterFive-inch K-style gutters or 6-inch half-rounds, the most common residential sizes, are able to handle the rainfall on most houses in most parts of the country. But houses with big, steep roofs or those located in climates prone to heavy downpours may need wider gutters and extra downspouts to keep rainwater from overflowing.

To figure out what size gutters you need, first you’ll need to calculate the square footage of the gutter’s drainage area. For a simple gable-end roof, you would only need to make two calculations, one for each slope. Hip roofs and intersecting roofs have multiple facets, and for those you’ll need to add up the area (length x width) of each surface within a drainage area to get the total square footage.

Adjusting for Pitch and Rainfall

Once you know the total square footage of drainage for each gutter, you’ll need to adjust for the following two factors:

1. Roof-pitch factor

roof pitch chart

The steeper a roof’s pitch, the more windblown rain it can collect. You can measure pitch with a 2-foot level and a tape measure: Hold one end of the level against the roof, level it, and then measure the distance between the roof and the underside of the level at its midpoint, which gives you a 12-inch run. A 5-inch gap, for instance, is a 5-in-12 pitch. Once you know pitch, you can find your roof-pitch factor in the table below.

Free Estimate

Roof pitch / Roof-pitch factor:
12 in 12 or higher 1.3
9 in 12 to 11 in 12 1.2
6 in 12 to 8 in 12 1.1
4 in 12 to 5 in 12 1.05
Flat to 3 in 12 1

2. Maximum rainfall intensity

The U.S. Weather Bureau records the maximum rainfall that could possibly happen in a 5-minute period, in inches per hour, for various regions. The higher the amount, the bigger a gutter has to be to keep from being overwhelmed in a storm burst. Download this handy table to find out the number for your area.

Sizing the Gutters

k style gutter and half round gutter measurementsMultiply the drainage area by the roof-pitch factor and rainfall intensity to find out the adjusted square footage. Then use the chart below to see what size gutter you need. (If a roof’s various drainage areas call for different gutter sizing , go for the biggest one.)

5-inch 5,520 square feet

6-inch 7,960 square feet

5-inch 2,500 square feet

6-inch 3,840 square feet

For example: A house in Chicago has a roof whose actual drainage area is 1,000 square feet. The 6-in-12 pitch factor (1.1) multiplied by 1,000 yields an effective area of 1,100 square feet. Multiplying that number by the local maximum rainfall intensity (6.8 inches per hour) yields an adjusted square footage of 7,480 square feet. Therefore, this roof should be equipped with 6-inch K-style gutters.


Gutter Capacity

One foot of 5″ K-Style gutters holds 1.2 gallons of water.

One foot of 6″ K-Style gutters holds 2.0 gallons of water.

Extra Capacity

What if the runoff is off the chart for standard gutters? You have three options:

1. Get 7- or 8-inch gutters. They’ll cost more and probably require a custom order through a professional installer.

2. Increase the pitch of the gutter. The standard is about ¼ inch per 10 feet. Increasing the pitch increases a gutter’s handling capacity, but the gutter may look askew over a long run.

3. Add downspouts. The above recommendations assume that you have properly sized downspouts every 40 feet. As with gutters, a downspout’s capacity must match or exceed the expected runoff. Use the chart below to figure out how many extra downspouts you need. Adding a 2 by 3 rectangular downspout, for instance, boosts your gutter’s capacity by 600 square feet of drainage area.

2 by 3 inches = 600 square feet
3 by 4 inches = 1,200 square feet

3 inches = 706 square feet
4 inches = 1,255 square feet

Information courtesy of the Sheet Metal and Air Conditioning Contractors National Association (SMACNA), a standards-setting organization based in Chantilly, Va.

Gutter Expansion

The subject of expansion is of interest to many installers. The chart below is a guide to help you see at a glance the effect of expansion on various materials.

gutter expansion

16 thoughts on “The Definitive Guide to Gutter Sizing”

  1. Marion says:

    We built our home in 2014. Who is responsible for determining the pitch of roof for gutters? Builder? Gutter company?

    1. Storm Master Gutters says:

      It depends on the many factors, but typically the gutter company determines the pitch of the gutters when installed.

  2. Thomas Steffens says:

    Can I get away with four inch. gutters on my 30×60 pole barn?

    1. Storm Master Gutters says:

      It really depends on some other factors. Is the barn is 60’ long and the end of roof to it’s peak is 30’ in length? What kind of roof is on the pole barn? Metal or asphalt shingles? Is the area around the barn treed or does it sit in an open field? It’s best to give us a call 856-391-0841 and one of our gutter pros can assist you.

  3. MDH says:

    I have one side of a shed that needs guttering that is approximately 12″ and then 2″ around to the back for the down spout. Please give me some recommendations and where to buy. I am in Arlington. TX. I would also prefer it in brown.
    Thank you,

  4. TrophyHomeRestoration says:

    Very impressive .. I really appreciate your work, you explained it very well.

  5. iconoclasthero says:

    Where is this this?!

    “The U.S. Weather Bureau records the maximum rainfall that could possibly happen in a 5-minute period, in inches per hour, for various regions. The higher the amount, the bigger a gutter has to be to keep from being overwhelmed in a storm burst. Download this handy table to find out the number for your area.”

  6. John Adams says:

    what is the size from the gravel stop to the top edge of the gutter on the outside edge. please info on a so called 5″ and 6″. thank you

  7. Gill hibbert says:

    Hi there, I have just moved into a
    Bungalow and the gutters just keep on overflowing. I have dealt with any blockages or debris and they are all clear. Do you think I need deeper guttering? Thanks

  8. Ethel says:

    Gutters and installation quote (half round gutters)

  9. Ethel says:

    Needs gutters installation on renovated house in Rietfontein

    1. Storm Master Gutters says:

      Hi Ethel, Please call us at 856-363-3388 for a free estimate. Thank you.

  10. Mike Gattshall says:

    What formula was used in calculating the expansion of aluminum?
    I ask because those numbers seem low to me.

  11. Mark Weber says:

    Thanks for the info that you’ve provided, it’s a great resource!

    Would you be able to recommend a gutter contractor in the north San Diego county area?

    Thanks, Mark

  12. Al says:

    What is the largest size downspout available for a 6 inch K gutter?

    Can 4 X 5″ downspouts be used with a 6 inch K gutter? I would think the answer would be ‘No’, because the 4 inch depth of a 4 X 5″ downspout would exceed the smaller depth of a 6 inch K gutter.

  13. Trey says:

    The 5-min rainfall intensity in Chicago is 0.68in/hr not 6.8in/hr. The effective square footage of the 1,000sqft roof should be 748sqft then, right?

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