Find Your Perfect Gutters Size in 3 Easy Steps

When looking for a new gutter system, it’s important to find out what size best fits your home and weather conditions. Improperly sized gutter systems can cause water to not shed properly and back up, allowing water to rot the fascia board or possibly enter the home. Most gutters come in five or six inches depending on the pitch and size of your roof and how much rainfall you receive. However, if you own a large home in a region with heavy rainfall, you may need a gutter system as large as seven or eight inches. Follow our step-by-step guide to find out how to determine what size gutters your home needs! 

Measure Your Gutter’s Drainage Area

First, you’ll need to calculate the square footage of the gutter’s drainage area by multiplying the length by the width of each of your roof’s facets. This is a simple process for a garble-end roof, as the area only needs to be found twice, once for each slope. It’s more complicated for hip roofs and intersecting roofs, as they have multiple facets. Work through the equation for each surface area, and then add these outcomes together to find the square footage of the gutter’s drainage area.

Measure Your Roof’s Pitch Factor

The pitch factor, or the number you use to find the true surface area of a pitched roof, is an essential calculation for figuring out the proper gutter size. The steeper your roof is, the more water will shed and at a faster speed. To calculate the pitch factor, you will need a two-foot level and a measuring tape.

First, you’ll need to measure the run, which is the horizontal distance between the roof ridge and the wall of the building. Hold the level horizontal with one end touching the roof’s surface, then measure 12 inches along the level from the end that’s touching the roof. This gives you the “run.” Next, you’ll measure the “rise” of your roof by measuring the space from the roof to the bottom of the level. Once you have those measurements you can determine the “roof pitch,” which is the ratio of rise to run.

So, if you have a “rise” of four and a “run” of 12, the roof pitch would be 4:12.

From there, you can use the table below to determine the pitch factor from the roof pitch. In this example, with a roof pitch of 4:12, the pitch factor would be 1.05.

Roof pitch                                                  Roof-pitch factor

12:12 or higher                                                 1.3

9:12 to 11:12                                                   1.2

6:12 to 8:12                                                      1.1

4:12 to 5:12                                                      1.05

0:12 to 3:12                                                      1.0

Determine Your Region’s Rainfall Intensity

Once you have determined the roof’s pitch factor, you will need to find the maximum rainfall intensity for your region. Maximum rainfall intensity is the most rain that can fall in five minutes, inches per hour. The higher your region’s maximum rainfall intensity is, the bigger the gutter you may need to handle the amount of water. The U.S. Weather Bureau collects rainfall intensity for specific regions, which comes in handy to determine your gutter size. You can find that information here.  

Calculate the Adjusted Square Footage

We’ve made it to the last calculation! Calculate the adjusted square footage by multiplying the square footage of the gutter’s drainage area by the roof pitch factor and the rainfall intensity. This number will tell you what size gutter you’ll need. For example, if your gutter’s drainage area is 1,000 square feet, your pitch factor is 1.1, and the rainfall intensity in your area is 6.8 inches per hour, then your adjusted square footage will be 7,480 (1,000 x 1.1 x 6.8 = 7,480).

Once you’ve calculated the adjusted square footage, all that’s left is to use the guide below to determine which size gutter system you need. In this example, with an adjusted square footage of 7,480, you would need a 6-inch, k-style gutter system.


  • Five-inch is large enough for 5,520 square feet
  • Six-inch is large enough for 7,960 square feet


  • Five-inch is large enough for 2,500 square feet
  • Six-inch is large enough for 3,840 square feet

Replace Your Gutters with Ferguson Roofing Today!

Determining the proper gutter size is a relatively easy process, but you don’t have to do it yourself! The experts at Ferguson Roofing have you covered. We’ll make sure your gutters are properly secured, pitched, and are the right size for your home. Plus, our complete gutter system replacements come with ten-year manufacturer and workmanship warranties. For a free multi-point home inspection and assessment, contact us today!