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Chimney Liner Size Guide

Measuring your chimney liner is an important task that will have a direct impact on your home. Unlike picking the wardrobe or a fridge, installing the wrong chimney liner will likely lead to venting problems, increasing the risks of toxic gases escaping into your home or damaging your chimney construction by heat or moisture.

At flue-pipes.com, we strive to help our customers, and understanding what size chimney liner you need is the first step in upgrading your chimney.

chimney on the roof

What Is a Chimney Liner and Why Do You Need One?

A chimney liner, also known as a flue liner, is a pipe that is designed to direct the smoke from the chimney to the outside of your home, and it also protects the walls from corrosion and heat damage.

If you are in the middle of a renovation, purchasing a new chimney with a compatible flue liner is also advisable so that your family’s safety is guaranteed. If you’ve used a chimney liner for a decade or more, changing it will also improve the performance of your furnace.

What Size Chimney Liner to Choose: The Basics

When choosing a chimney liner, there are three factors that matter: grade, diameter, and length. We will cover each of these in detail below and attach calculations of the flue liner sizes designed for various types of pipes.

Pick the Correct Grade

The grade of the chimney liner directly depends on the appliance it will be attached to. There are two types of grades used on gas and multi-fuel appliances:

  • 316L grade - This grade is suitable for gas and wood stoves. A 316 chimney liner will allow exhaust to smoothly exit the chimney and will prevent toxic gas leakage.
  • 904L grade - This grade is suitable for multi-fuel appliances, such as wood and coal furnaces. A 904 chimney liner has a twin-skin build that can withstand higher temperatures.

Measuring the Diameter

Once you know which grade you need, it will be easier to choose the next criterion – the diameter. The diameter is measured by the heat output or the width of a connector collar of your furnace. If possible, have a look at the manufacturer’s instructions to find the precise diameter the size of your chimney outlet.

Gas appliances can be equipped with a 5” or 6” chimney liner. When choosing the correct chimney liner size, make sure the inside opening of the flue is the same diameter as your appliance’s outlet.

flexible flue liner

5” Flexible Flue Liner

6” Flexible Flue Liner

Multi-fuel appliances, on the other hand, usually have bigger diameters, about 7”, 8”, or 9”. However, DEFRA-approved furnaces and stoves mostly have a smaller diameter of 5”.

Depending on the type of furnace, the collar may be slightly smaller than the diameter of a flue liner on purpose, so we advise you to purchase an adaptor for a smooth installation.

single wall to flex adaptor

Single wall 5" to Flex 6" adaptor

Single wall 4" to Flex 5" adaptor

Choosing the Length

Determining the length is the final step of measuring your chimney liner size. The fastest and most accurate way to find out the length you need is to ask the nearest chimney sweep to do it before they clean the appliance. If you want to DIY the project from the beginning to the end, you can drop the measuring tape from the chimney crown all the way down to the opening.

Make sure the length is measured accurately because a chimney liner must reach the top of your chimney and not shorter. To be on the safer side, you can always purchase a longer chimney liner and cut the excess part.

At no point should you try to connect two separate chimney liners, as this may lead to undesired complications when you use your appliance. The connection can only be possible with a special adapter.

Flue Pipes lets you buy chimney liners by metre and with a lifetime warranty. Provide us with accurate measurements and collect the pre-cut flue liner from us.

Chimney Liner Size Calculator

To know what chimney liner size you need, you can measure the height and width of your chimney inside your home and divide by either 10 to 12 or 10 to 1 ratio. A 10-to-1 ratio should be applied for appliances shorter than 25’, and a 12-to-1 ratio is for appliances longer than 25”.

Here are the calculations (with a 10-to-1 ratio applied) for fireplace openings of various sizes in square inches and their equivalent liner diameters:

Fireplace Opening

Liner Diameter

Up to 600 sq. in.


601 - 760 sq. in.


761 - 942 sq. in.


943 - 1140 sq. in.


1141 - 1356 sq. in.


1357 - 1846 sq. in.


When it comes to gas and multi-fuel appliances, the best recommendation is to follow the manufacturers’ guidelines. Most of the charts you find on the internet may differ from the size your stove or furnace needs.

Final Word

Choosing the correct size of your chimney liner prevents poor draft and creosote buildup. In addition, the performance of your appliance will be noticeably poorer. Hence, following this guide will help you in measuring your chimney and purchasing the chimney liner that fits perfectly.

If you’re looking for a durable chimney liner, Flue Pipes offers 316 and 906-grade chimney liners that come in 5” and 6”. Ideal for wood, gas, and other appliances, our chimney liners are easy to customise by cutting and installing, even without professional assistance.

If you have any questions or need some advice regarding a chimney liner, feel free to ask any technical questions through our live chat or by calling us.