Steps Toward Choosing Proper New Rain Gutter Size, Part 2

In part one of this two-part blog series, we went over some of the primary factors involved in choosing the right rain gutter size for your home. Ensuring your gutters are properly sized is vital within the process of installing new gutters, as it will protect you from overflow or leakage issues that could create water damage and foundational concerns in some cases.

At Wizard Rain Gutters, we’re happy to go through every detail of gutter selection with you as part of our rain gutter installation services, including helping you understand the available sizes and which is best for your home. In today’s part two, we’ll go over some of the final factors involved, plus how to combine each of them into a final calculation – we’ll even go over a basic example that will help you understand this in practice.

choosing proper rain gutter size

Potential Rainfall Intensity

Along with drainage area and roof pitch, areas we discussed in part one, it’s also important to consider the potential maximum rainfall intensity that’s possible within your area. This is a metric that’s calculated based on a five-minute interval of time, with the US Weather Bureau taking the lead here.

Maximum rainfall intensity will be defined as inches of rainfall per hour. As you might imagine, the higher the number of your maximum rainfall intensity, the larger gutter you will require to avoid overflow issues in case of larger storms. Rainfall intensity varies by region – you can use this basic table to understand the maximum potential intensity in your area, helping you decide on the proper gutter.

Making Calculations

Let’s assume you’ve already found the proper measurements for each of the three factors we’ve discussed so far: Drainage area, roof pitch factor and maximum rainfall intensity in your area. Now it’s time to make the simple calculations needed to determine the proper gutter.

To do this, simply take the square footage of the drainage area and multiply this by the roof-pitch factor. Then, multiply this by the maximum rainfall intensity. You now have a number called “adjusted square footage,” and it will be your guide to the proper gutter size. A basic list of gutter types and the square footage that works best for each size:

  • 5-inch K-style gutter: 5.520 square feet
  • 6-inch K-style gutter: 7,960 square feet
  • 5-inch half-round gutter: 2,500 square feet
  • 6-inch half-round gutter: 3,840 square feet


Let’s say your home has a roof with a drainage area of 1,000 square feet, plus a pitch factor of five-in-twelve at 1.05. We multiple 1.05 by 1,000 to give ups 1,050 feet, then multiply that by your local maximum rainfall intensity – for the sake of argument, let’s say that’s 2.7 inches per hour. This brings us to an adjusted square footage of 2,835 feet, which suggests we should select five-inch half-round gutters based on the chart above.

For more on choosing the proper gutter size, or to learn about any of our rain gutter replacement or repair services, speak to the staff at Wizard Rain Gutters today.