Gutter and Downspout Sizing: The Ultimate Guide

Figuring out the perfect gutter size and downspouts can be tricky. You don’t want to go too small or the water won’t drain properly. Go too large and you may end up overspending on gutters that are overkill.

Let’s jump into the guide on choosing the perfect gutter size.

What Size Gutters Do I Need?

5 inch vs 6 inch gutters k-style

To choose your gutter size, there are few different methods. It’s something that can get complicated fast, especially if you’re not a professional gutter expert.

Luckily, it can be a simple as you want it to be.

Method #1: Check What Size Gutters your Neighbors Have

If you want the easiest way to pick the right gutter size, just look at your neighbor’s homes to see what they’re using.

There is most likely a standard in the area for gutter size in the area that you live in. Keep in mind, there can be more factors at play, such as if you have a large roof, or if you’re roof has a more intense pitch or angle to it.

But if you have some common sense, looking at what your neighbors have is usually the easiest option.

Method #2: Search What Gutter Sizes are Common for your Area

Another thing you can do is look up the standard in your area with a simple Google search, such as gutter size for “your city”. But if you live in a small area, you may struggle to get results for this search.

Method #3: Measure your Roof Pitch, Surface Area, and Rainfall Per Hour

The last way is to use a calculation to determine the right gutter size, such a roof pitch, roof surface area, and the amount of rainfall in your area.

This is definitely the nerdiest way and what we’ll focus on in this article. We’ll go over how to choose the right gutter sizing and type based on these factors.

Later on, we’ll also talk about downspout sizing, so you can read that as well if you’re interested.

Also if you want specialty gutters in certain shapes, you may struggle a bit more to figure out the right gutter size, but we’ll get into that too.

Choosing the Right Gutter Style

When it comes to the wild world of gutters (crazy stuff right?), there are two main gutter types: K-style and Half-round gutters. The difference between these two types is the shape: K-style and half-round.

Shape plays an extremely important role in how much water the gutters can manage water flow and in terms of how they look. That being said, there are more factors at play as well, but we’ll get into that later.

Let’s get into the differences.

K-Style Gutters

k-style gutters

K-style gutters are the go-to nowadays for anything gutter. Why? Well, the shape of them is superior for carrying more water. In fact, K-style gutters can carry 40% more water than half-round gutters in a comparable size.

This has to do with the deeper shape that can allow for more rainfall to be funneled through without overflow.

K-style gutters get there name from the shape of the profile (how the gutters look from the side), some people think they are in the shape of a K. Although I don’t really see the resemblance, we’re kind of stuck with the naming convention aren’t we?

This gutter type is what most contractors are familiar with using and are manufactured at scale. Contractors will usually have equipment specifically made to install this type of gutter, so because of that, they are usually cheaper.

Plus K-style gutters are usually seamless, making them less susceptible to leaks and degradation over time. You can read more about in this article: seamless vs regular gutters.

Half-Round Gutters

Half-round gutters are cut in the shape of a semi-circle, or a circle cut in half. They are commonly found on older homes and many enjoy the “look” of them as they are simple and aesthetic.

On older homes especially they can boost curb appeal and even increase home values.

However, they come with a few downsides. The first of which being, they simply don’t work as well as K-style gutters. When comparing overall water-flow at similar sizes, they lose every single time.

They will typically be more expensive as well, as contractors are not used to installing this shape of gutter. One of the advantages, however, is that half-round gutters tend to get clogged less as the shape makes it easier to wash all of leaves and gunk away when it rains.

That being said, half-round gutters will still need regular maintenance to avoid common issues such as overflowing.

Other Gutter Types

While most only talk about the two common gutter types, half-round and k-style, there are other options out there. Gutters are usually just panels of aluminum (or vinyl) that are bent into the proper shape. So in reality, gutters can be made into any possible shape.

While they are uncommon to find, there is actually a crazy amount of variety when it comes to gutters. Similar to K-style, you can get:

  • A-style
  • B-style
  • C-style
  • …continue until L-style

Each one is a different shape and offers a variety of benefits. Most tend to go with K-style simply because they are made for maximum waterflow and have a decorative flair to them that almost looks like crown molding.

Things to Consider When Sizing Gutters

When choosing your gutter sizes, you want to look at a variety of elements to make the right choice. If you’re home has an extremely angled roof pitch, you get a lot of rainfall, or you simply have a large roof, larger gutters will be the way to go. You can read more about gutter sizing in this article about 5-inch vs 6-inch gutters.

Roof Pitch

measuring the pitch of roof with level and measuring tape
Distance MeasuredPitch Factor
3 inches or less1
4-5 inches1.05
6-8 inches1.1
9-11 inches1.2
12 or more inches1.3
A pitch factor of over 1.1 is considered steep.

The pitch of your roof is one of the biggest factors in determining gutter size. Why?

If you’re roof has a more intense pitch (the angle of the roof), naturally, water will fall faster down the roof and into the gutters. So it makes sense that the greater the roof pitch, the bigger your gutters need to be.

Imagine going down a slide that’s very steep vs a flatter slide, on the steeper one you’ll fly down it much faster. Well, rainwater works the same way with your roof.

To figure out if you have a steeper roof than normal, you can do the tried and tested act of peeking over at your neighbor’s houses. If they have a similar roof, you probably need similar gutters.

Or if you want to get more technical, you can actually measure the slope of your roof to give you a better idea of your actual roof pitch.

This video will walk you through how to determine roof pitch in a super simple guide. Note: Keep in mind you’ll need to climb on your roof to do this, so normal safety precautions are advised.

Heavy Rainfall

Gutter size (K-style)2 inch/hr3 inch/hr4 inch/hr5 inch/hr6 inch/hr
5 inch1250 sqft834 sqft625 sqft500 sqft416 sqft
6 inch1920 sqft1280 sqft960 sqft968 sqft640 sqft
Maximum roof size (in square feet) based on rainfall by the hour for each gutter size. Calculations taken with a roof at an incline at 1/16.

If you’re in an area that experiences heavy rainfall, that’s one of the biggest signals that you’ll need larger gutters. Unfortunately, we can’t just take the average rainfall your area gets and use that because we’re looking at the rate of rainfall per hour.

This is important because you could be in an area that gets a lot of rain, but the rain falls in a very steady slow way. If you get a light trickle that doesn’t stop for days at a time, you probably don’t need larger gutters because the gutters will never overflow. Even though on paper, it looks like you get a lot of rainfall.

Instead you need to look at larger gutters if you experience big bursts of rainfall. Especially in places where you may experience more intense weather events like monsoons or tropical storms where it rains cats and dogs for a short period of time. That sort of weather is much more likely to overwhelm your gutters and lead to overflowing.

In that case, larger gutters is the way to go.

To figure out your rainfall per hour in your area, you can do a simple google search, such as: “your city” + rainfall per hour.

Keep in mind, global warming is contributing to more extreme rainfall year over year, so if you’re on the edge between choosing the gutter size, it could be a good idea to go a size up.

3. Roof Surface Area

pitch of roof for rainfall along with height and width. Helps choose between 5 inch and 6 inch gutters.

Do you have a big house with a large roof? Well, that can impact the size of your gutters. A bigger roof equals more water that your gutters need to catch and drain away from your house.

So naturally, the surface area of your roof’s plane will also impact the size of your gutters.

A larger roof means more rainfall entering your gutters.

Your “roof’s plane” refers to the amount of roof surface area your gutters have to support.

This is a simple measurement to figure out. Take the distance from the start of your gutters to the peak of your roof. Then measure the overall distance of your gutters for that section of roof.

It will look like: roof length x gutter width = roof surface area.

This measurement will help you determine the gutter size and the number of downspouts you will eventually need.

A larger surface area = bigger gutters, smaller surface area = smaller gutters.

Note: A good rule of thumb is less than 1000 square feet can be supported by 5-inch gutters and more than 1000 square feet will need 6 inch gutters. For larger gutters than 6 inches, you’re mostly looking at commercial properties and not residential homes.

While roof surface area, pitch, and rainfall are the main factors that determine gutter sizing, there are other factors that may impact the decision between gutter sizes which we’ll get into soon.

How to Size and Place Downspouts on Your Gutter System

Downspout Placement Rule: Gutter downspouts should be placed 25-35 linear feet apart, with a maximum distance of 40 feet.

After determining your gutter size, naturally you will need to figure out what size downspouts you need, how far apart to place them, and how many you need.

The general rule of thumb is to place downspouts 25 to 35 feet apart with a maximum of 40 feet between each downspout. For a full guide to figure out how many downspouts do you need, check out this article.

Let’s walk through it, just keep in mind it will depend on the gutter type you choose.

Sizing Downspouts for K-Style Gutters

Gutter TypeGutter SizeDownspout Size
K-Style5 inches2 inch x 3 inch
K-Style6 inches3 inch x 4 inch
K-Style6+ inchesCustom sizing
Half-Round5/6 inches3/4/5 inches*
*Downspout sizing with half-round gutters depends on total gutter drainage capacity. Keep in mind, half-round gutters do not have as high of water capacity as K-style gutters.

Chances are your home has K-style gutters or “ogee” gutters.

K-style gutters have that name due to the shape of the profile somewhat resembling the shape of a K. They are considered the gold standard for water drainage as they can hold more water than other styles.

For 5 inch K-style gutters, you’ll need a 2×3 inch downspout. For 6 inch K-style gutters, you’ll need a 3×4 inch downspout.

If you have even larger gutters than 6 inches, you’ll need a custom downspout size to support that amount of water flow.

Sizing Downspouts for Half-Round Gutters

Half-round gutters are less common, but homeowners like them due to the visual appeal.

They are, well, half-round shaped or a semi-circle. This style of gutter does not drain quite as much as K-style, so the downspout sizing will depend a lot more on your specific situation and there is no cookie-cutter sizing guide.

Because of the shape, half-round gutters use rounded downspouts to match the half-round “look”. Because most half-round gutters are 6 inches due to poor water flow, the downspouts will typically be a similar size. But there are options in the 3 to 6 inch range.

Ultimately, it depends on the rainfall in your area. If you are not sure about what size to choose, it’s always a good idea to get a free quote from a local contractor and probe them for information about what works in your area.

How Many Gutter Downspouts Do You Need?

To get an idea of how many downspouts you need, you’ll need to figure out the total linear foot of gutter that’s getting installed. You can do this by measuring your roofline or eaves with a measuring tape in places that you want gutters to be installed. Then you add at least downspout every 25-35 linear foot based on the downspout placement rule we talked about earlier.

That should give you a good idea about how many downspouts you’ll need for your project.

Just keep in mind, there may be more nuance to how you actually place the gutter downspouts.

Some homeowners don’t like the look of having gutters in their front yard or it may not be allowed by your local Homeowners Association, in which case, the water will need to be routed to the sides of your home before draining.

Also you may have small sections of gutters that aren’t connected to the rest of your gutter system and that area will need it’s own independent downspout. This is more common on two-story homes that have some sections that are also one-story. You’ll end up with small sections of gutter here and there that will need a downspout.

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