How to Determine the Right Gutter Size

Ready to install gutters but aren’t sure which size to use? It’s not too difficult to determine the right gutter size for your home. However, there are several calculations, so let’s break it down:

Two gutter sizes

Primarily, there are two different sizes of gutters for standard homes:

  1. 5-inch K-style gutters
  2. 6-inch half-round round gutters

Either option should work for your home, but houses with large or steep roofs may want to consider the 6-inch half-round gutters to prevent overflowing. 6-inch gutters can handle almost 50% more water than 5-inch gutters.

Calculate drainage area to determine gutter size

For a simple gable roof, you need to make two calculations – one for each slope. If your roof has multiple facets, the area of each surface that has a drainage area needs to be calculated to know the entire square footage of the drainage area.

Use the roof-pitch factor to determine gutter size

The roof-pitch factor indicates the steepness of your roof and can be easily calculated with a two-foot level and a tape measure.

Place one end of the level against the roof, level it, then use the tape measure to measure the distance between the middle of the level and the roof underneath it. This measurement is called the “twelve-inch run”. For example, a six-inch gap between the underside of the level’s midpoint would be considered a six-in-twelve pitch.

Use this table to determine your roof-pitch factor:

Roof pitch                                                  Roof-pitch factor

Twelve-in-twelve or higher                               1.3

Nine-in-twelve to eleven-in-twelve                   1.2

Six-in-twelve to eight-in-twelve                        1.1

Four-in-twelve to five-in-twelve                        1.05

Zero-in-twelve to three-in-twelve                     1.0

Find the maximum rainfall intensity  

The United States Weather Bureau records the maximum possibly rainfall over a five-minute interval. The higher the number, the larger the gutter. Use this table to find the maximum rainfall intensity in your region.

Determine the gutter size

Once you have all the measurements, you’re ready to determine the right gutter size for your home. Take the square footage of the drainage area and multiply it by the roof-pitch factor, as well as the maximum rainfall intensity. This number is called the adjusted square footage, and you can use it with the chart below to determine the recommended size of gutter for your home. If different drainage areas on your roof call for different sizes of gutters, choose the one that is the largest.


  • Five-inch 5,520 square feet
  • Six-inch 7,960 square feet


  • Five-inch 2,500 square feet
  • Six-inch 3,840 square feet

Don’t forget the downspouts!

Don’t forget to factor in the downspout location when you’re determining your gutter size. Additionally, consider installing leaf screens to maximize the life of your gutters.

Call Good to Go Gutters today!

Calculating the size of your gutters can be tricky. Give us a call for your free estimate and we’ll be happy to give you a hand. Good to Go Gutters is an expert in gutter installation in the south metro area and we treat every home as if it’s our own. We offer quality work and a quick turn-around.