What Size Chimney Cap Do You Need?

Finding the right sized chimney cap can be challenging. There are so many questions to consider: how do you measure your chimney flue? Are single-flue and multi-flue chimney caps sized differently? What are the standard sizes, and what if your chimney doesn’t fit them?

Trust us, we know it’s confusing. We manufacture chimney caps at HY-C, and we have to make multiple styles in dozens of sizes in order to help cover as many chimneys as we can. It’s a lot to keep straight — and in this guide, we’re going to simplify the process for you.

By the time you’re done, you’ll be able to narrow your choices down to figure out exactly what type of chimney cap you need. You’ll also know what sizes of chimney caps are available in that type, and how to measure your chimney or flue to ensure the perfect fit.

Table of Contents (click to jump to a section):

• Square Single-Flue Chimney Cap Sizes
• Rectangular Single-Flue Chimney Cap Sizes
• Round Single-Flue Chimney Cap Sizes
• Multi-Flue Chimney Cap Sizes
• Band-Around-Brick Chimney Cap Sizes

Types of Chimney Caps

There are several styles of chimney caps made from a variety of different materials. From a sizing perspective, though, it’s helpful to group chimney caps into three simple categories:

• Single-flue
• Multi-flue
• Band-around-brick

Single-flue chimney caps can either be rectangular, square, or round. These caps attach directly to a chimney’s flue tile via four bolts. If your chimney only has one flue, one of these caps is your best bet.

A single-flue chimney cap installed on a chimney flue

Multi-flue chimney caps are either square or rectangular. Instead of attaching to the chimney’s flue tile, a multi-flue cap is bolted directly onto the chimney’s crown. If your chimney has two or more flues, you can either attach a single-flue cap to each flue, or you can cover all your flues at once with a multi-flue cap.

A multi-flue chimney cap installed on a chimney crown

Band-around-brick chimney caps get their name because they clamp onto a chimney’s top row of brick by way of a metal band. These square or rectangular caps are adjustable by tightening or loosening the four bolts in each corner (for a total of 16 bolts per cap). Band-around-brick caps are usually used to cover a single chimney flue; they’re typically not big enough to cover two or more flues (unless you order a custom-sized band-around-brick cap).

A band-around-brick chimney cap installed on the top row of a chimney’s brick

Consider these three categories and determine which one is best suited to your chimney flue and/or crown configuration. Once you decide on a style, it’s time to find the right size.

Single-Flue Chimney Cap Sizes

Square, rectangular, and round single-flue chimney caps are available in quite a few sizes, and each of those sizes fit a range of flue dimensions. The first step in determining what size chimney cap you need is to find out if your flue pipe is square, rectangular, or round.

Square Single-Flue Chimney Cap Sizes

If you have a square chimney flue, there are 6 single-flue size options available to you. Chose the cap size you need based on the dimensions of your square chimney flue tile:

How to Measure Your Square Flue Tile

To find out which square single-flue chimney cap you need, measure the length of your flue tile from outer edge to outer edge.

This particular flue tile measures 8.5” x 8.5” from edge to edge. Using the table above, this flue’s measurement falls into the 7.5” x 7.5” to 9.5” x 9.5” range, meaning the flue will require a 9” x 9” chimney cap.

Rectangular Single-Flue Chimney Cap Sizes

If you have a rectangular chimney flue, there are 5 single-flue size options available to you. Chose the cap size you need based on the dimensions of your rectangular chimney flue tile:

How to Measure Your Rectangular Flue Tile

To find out which rectangular single-flue chimney cap you need, measure the length and width of your rectangular flue tile from outer edge to outer edge.

This particular flue tile measures about 8.5” x 13” from edge to edge. Using the table above, this measurement falls into the 7.5” x 11.5” to 9.5” x 13.5” range, meaning this flue will require a 9” x 13” chimney cap.

Round Single-Flue Chimney Cap Sizes

If you have a round chimney flue, there are 6 single-flue size options available to you. Chose the cap size you need based on the diameter of your round chimney flue tile:

How to Measure Your Round Flue Tile

To find out which round single-flue chimney cap you need, measure the diameter of your round flue tile from outer edge to outer edge.

This particular flue tile measures about 11.75” in diameter from edge to edge. Using the table above, this measurement falls into the 11.5” to 12.5” range, meaning this flue will require a cap with a 12-inch diameter.

Multi-Flue Chimney Cap Sizes

Chimneys that have multiple flues vary in size much more than chimneys with just one flue. As a result, multi-flue chimney caps are available in a wide range of sizes. There are 3 different sizes of square multi-flue chimney caps:

While square multi-flue caps are pretty straightforward, the majority of multi-flue chimney caps are rectangular. There are 14 rectangular multi-flue chimney cap sizes:

You should note that there are a few different ways to measure a multi-flue chimney cap:

• You can measure from screen to screen
• You can measure the size of the cover (or “hood”)
• You can measure from flange to flange

The “screens” are simply the mesh screens that act as the “walls” of the cap, and the “cover” is the top portion of the cap that actually covers the flue.

The “flanges” are the pre-drilled metal edges that stick out of the cap perpendicular to the screens. The screws that hold the cap on the chimney crown go through the pre-drilled holes on the flanges and into the crown.

When measuring a multi-flue chimney cap for installation, it’s most helpful to consider the screen-to-screen size (shown in the charts above) and match it to the minimum chimney crown size needed for installation.

How to Measure Your Chimney Crown for a Multi-Flue Chimney Cap

To find out which multi-flue cap size you need, measure the length and width of your chimney crown from its flattest point on one edge to its flattest point on the opposite edge. Do this both for the length and width of your crown.

The crown of the chimney above is square — 18” x 18”. The table above indicates that a crown measuring at least 17.25” x 17.25” needs a square, multi-flue chimney cap with flanges measuring 15.25” x 15.25”.

It’s also important to ensure that the flanges rest at least one inch from the flat edge of the chimney crown. Inserting screws too close to the edge of your crown could cause it to crack, resulting in damage to the crown over time.

Make sure the cap’s flanges are at least one inch from the edge of your crown

Band-Around-Brick Chimney Cap Sizes

Band-around-brick chimney caps are essentially single-flue chimney caps that offer more coverage. Instead of bolting directly onto the flue tile, these caps clamp onto the top row of the chimney’s brick, providing more robust protection for the concrete crown of the chimney.

These chimney caps come in 4 different sizes — 3 for square chimneys, and 1 for rectangular chimneys:

How to Measure for a Band-Around-Brick Chimney Cap

In order to find out which size band-around-brick chimney cap will best fit your chimney, start by determining whether you have a square or rectangular chimney. From there, measure the length and width of the top row of your chimney’s brick.

This particular chimney is square, and it measures about 17.5” x 17.5”. Using the table above, this measurement falls into the 16.5″ x 16.5″ to 18.5″ x 18.5″ range, meaning this flue will require a square size 2 chimney cap.

What if Your Chimney Doesn’t Fit the Standard Sizes?

Before now, you probably had little to no idea of how to measure a chimney cap for installation. And it’s no wonder it’s so confusing — the tables above indicate that there are 38 possible sizes of chimney caps, and that’s just what we were able to cover here!

Once you start to narrow the list down based on the shape of your flue or how many flues you need to cover, finding the correct size becomes a much simpler process.

There’s a lingering question though: what if your chimney cap doesn’t fit any of the sizes mentioned in this guide?

If that’s the case for you, don’t worry — that just means you need a custom-made chimney cap. We make plenty of custom caps at HY-C; in fact, since 2020, we’ve manufactured over 9,000 of them.

If you believe you need a custom-made chimney cap, our custom chimney cap guide covers all the styles, sizes, and materials we offer. Give it a look, and you’ll be one step closer to finding the perfect cap to cover your chimney.