Gutter and Downspout Sizes, and How to Choose Them

When it comes to gutters and downspout sizes, you can’t just eyeball the situation. Ensuring a proper fit that will prevent clogs, buildup, and moisture damage from affecting your roof requires careful measuring. Plus, downspout placement is just as important as downspout size for efficiently directing rainwater away from your roof. In this guide, we’ll help you figure out the best gutter size and downspout combination to protect your home, rain or shine.

Standard Gutter Sizes

Did you know that there are different gutter styles to choose from? Your gutter gauge or thickness should be sized based on the efficiency and capacity of the gutter style you choose.

K-Style Gutters

K-style gutters are the most common gutters you'll see on residential homes. You'll know them by their signature front edge that matches the home's crown molding. This option is praised for being durable, versatile, and easy to install.

If you're choosing K-style gutters for a roof area under 5,500 square feet, the recommendation is a 5-inch standard gutter size. For roof areas between 5,500 and 8,000 square feet, the recommendation is a 6-inch gutter.

Half-Round Gutters

Half-round gutters are more commonly used on historic homes. They can also complement classic-style new constructions. While half-round gutters offer aesthetic benefits, they generally don't move water as efficiently as K-style gutters.

For roof areas under 2,500 square feet, 5-inch half-round gutters are recommended. Roof areas between 2,500 and 3,850 square feet require 6-inch gutters.

Standard Downspout Sizes

The downspouts in a gutter system play an important role because they make sure water drains efficiently. The general rule about placing downspouts is that you need one downspout for every 25 to 35 linear feet of gutter. The maximum gutter length between downspouts is 40 feet. If you're in a high-capacity situation due to either roof pitch or rainfall levels, adding extra downspouts can boost your gutter's drainage capacity.

You should also install downspouts so that they are positioned to move rainwater away from your home's foundation. Downspouts that do not adequately drain away from your home's foundation will eventually cause water to seep into your basement and foundation. If you're unable to place downspouts in a way that directs water away from your home's foundation, you may need to utilize gutter extensions.

Sizing Downspouts for K-Style Gutters

If your roof's square footage requires 5-inch gutters, you'll need a 2-inch by 3-inch downspout. That creeps up to a 3-inch by 4-inch downspout for 6-inch gutters. It's necessary to create custom downspout solutions for gutters wider than 6-inches that are being installed to address excessive pitch or rainfall levels.

Sizing Downspouts for Half-Round Gutters

Round downspouts come in 3-, 4-, 5-, and 6-inch diameters. These downspouts are also available in a variety of colors to match your home. Contractors typically provide guidance on how to choose the right size based on the specifics of a home's roofing system. The 6-inch option generally works best for most homes.

How to Calculate Gutter Size

To figure out the gutter size you need, you can calculate the square footage of your gutter's drainage area. A basic gable-end roof will require calculations for both slopes. If you have a hip roof, you'll need to calculate the area of each individual surface that contains a drainage area. The equation for total square footage on every surface area of your roof is always length x width.

"If you have a hip-and-ridge roof, you are more likely to be able to install smaller gutters. This is because the water is distributed across more slopes than if it was a classic gable roof with only two slopes," says Jose Figueroa, Gutter expert at Artisan Quality Roofing.

Factors That Impact Gutter Size

Once you've calculated the square footage for each drainage section, it's time to focus on special considerations.

Rainfall

The National Weather Service refers to the number of inches of rain that can fall per hour based on a 5-minute sample period as a region's maximum rainfall intensity. If you live in an area with a high maximum rainfall intensity, you'll need larger gutters to prevent your system from becoming overwhelmed in heavy, sudden rains.

Seven-inch and 8-inch gutters are rarely used in residential buildings, though there are cases where they work—for example, an extraordinarily large house with steep roofing. A home that gets heavy rainfall should be fine with a 6-inch gutter.

Due to their extra weight, larger gutters require custom ordering and installation through a professional gutter installation pro near you. You may also be able to use traditionally sized gutters by having a professional increase gutter pitch during installation to increase capacity. However, you should make sure you install gutters in the right place—as close to the shingles as possible—to prevent expensive problems.

Roof Pitch

If your home has a steeper roof pitch, there's a high risk for windblown rain to collect in your gutters. You'll want to size up your gutters because rain will be running off the gutters at a faster pace compared to other roofs. Steep roof pitches—even smaller roofs—benefit from 6-inch gutters.

Roof Shape

You may be surprised to learn that your roof's shape also plays a role in how large your gutters should be. If your roof is designed in such a way that it creates a bottleneck of water in a particular area, you likely need larger gutters.

What to Do If You Suspect Your Gutters Are the Wrong Size

You can troubleshoot whether you have the proper size gutters by looking for certain signs. Keep an eye out for water spilling over the side of your gutters during rainstorms, even after you’ve cleaned them. You should also check for visibly sagging gutters, which may droop due to the excessive weight of water. Any standing water in the gutters, especially near your downspouts, is another clue that your gutters are too small. 

If you see any of these signs, call a gutter pro to assess whether your gutters are the right size. If they find that you need a different size, know that gutter installation costs $4 to $40 per linear foot.