How To Calculate The Size Of A Gutter

How To Calculate The Size Of A Gutter

The Ultimate Guide to Calculating Gutter Size for Your Home


Gutter size is crucial when it comes to gutter replacement and installation. Incorrect sizing can lead to various issues, such as clogging, water overflow, and even structural damage. To avoid these problems, it’s essential to accurately calculate the size of your gutter your home requires. This guide will walk you through the process, including the measurements and calculations required for optimal gutter sizing.

Importance of Correct Gutter Sizing

The Consequences of Undersized Gutters

Gutters play an important role around your home and if the guttering is undersized, it will not be able to function effectively. The size of your home, pitch of your roof and location of your home all has an important role, when selecting the size of your guttering.

The Drawbacks of Oversized Gutters

Oversized gutters are not only unsightly but also require larger downpipes. This increases the overall cost of gutter replacement, which is already an expensive undertaking. While it’s essential not to skimp on gutter quality, there’s no need to spend more than necessary.


Factors Affecting Gutter Size

The size of your gutter depends on factors like the surface area of your roof, the amount of rainfall in your region, the size of your house, and the pitch of your roof. In most cases, guttering for a simple roof design will be five or six inches across, with either a half-round or K-style pattern.


Measuring Your Roof for Gutter Sizing

Roof Surface Area Calculation

To determine the gutter size, you need to calculate your roof’s surface area. You can do this by measuring the length and width of each slope of the roof. For a simple roof with two sloping sides and gable ends, the process is straightforward. For more complex roof shapes, measure each section and add the areas together.

However, measuring the roof can be dangerous for amateurs, as it involves climbing ladders or scaffolding towers. An alternative and safer method is to measure the length of the walls on the ground and add in the overhang of the eaves.

Roof Pitch Measurement

The pitch of your roof also plays a role in gutter sizing. To measure the roof pitch, you can take a measurement inside the roof space, using a two-foot spirit level. Hold the level against the sloping roof and measure the distance from the one-foot point upwards to the inside of the roof. This measurement gives you the roof pitch (e.g., 9 inches at the 12-inch mark on the level means a 9-in-12 pitch).

Next, use the following table to determine the pitch factor for your roof:

Pitch Multiplier
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

Multiply the surface area by the pitch factor to find the rain collection area for your roof. This will help you choose the right size guttering.

Gutter Capacity Guidelines

In plastic K-style guttering, 5-inch gutters can drain up to 5,520 sq. ft., while 6-inch gutters can handle up to 7,960 sq. ft. For half-round guttering, 5-inch gutters have a capacity of 2,500 sq. ft., and 6-inch gutters can accommodate 3,840 sq. ft.

Gutter Repair Considerations

If you’re dealing with minor issues, gutter repair may be a more cost-effective solution. For plastic gutters, repairs often involve replacing perished seals or damaged sections. Cast iron gutters can frequently be patched using sealants, making repairs a viable option in many cases.


Determining the appropriate gutter size for your home is crucial for effective water drainage and long-lasting performance. While calculating gutter size may not be a simple task, understanding the process can help ensure you make the right choice. If you’re not confident in taking on the measurements and calculations yourself, consider hiring a professional gutter repair specialist to handle the job. By choosing the right expert, you can ensure your gutters are correctly sized and installed, protecting your home from potential water damage.