How to Measure for a Chimney Liner (Diameter, Length & Kit Checklist)

A stainless chimney liner turns an old masonry chimney into a properly sized, continuous flue for a new wood stove, insert or gas appliance. Ordering the wrong diameter or a liner that is a few feet short is the most common and most frustrating mistake. Here is exactly what to measure, and what else belongs in the kit.

What you need before you start

• The installation manual for the appliance that will connect to the liner (it lists the required flue diameter and any minimum chimney height).
• A tape measure, a flashlight, and a long measuring tape or weighted string for the chimney height.
• Safe roof access, or measurements from a chimney professional. Never work on a roof without proper fall protection.

Measurement 1: Liner diameter (from the appliance)

The liner diameter is set by the appliance, not the chimney. Most wood stoves and inserts use a 6″ flue; larger units may need 7″ or 8″; many gas inserts use a 3″ or 4″ co-axial liner kit specified by the manufacturer. Check the manual for:

• Required flue diameter. Never install a liner smaller than the appliance outlet.
• Whether oval liner is allowed if the round size will not fit your flue.
• Minimum and maximum chimney height for proper draft.

Measurement 2: Inside the existing flue (the fit check)

From the top of the chimney, measure the inside dimensions of the clay flue tile, not the outside. Common tile sizes are 8″ x 8″, 8″ x 12″ and 12″ x 12″ nominal, but the clear opening inside is smaller than the nominal size, so measure it.

These are planning guides only; tile joints, offsets and mortar can reduce the usable space. If the fit is tight, an oval liner (where the appliance allows it) or a professional assessment is the safe route.

Measurement 3: Liner length

1. Measure from the top of the chimney crown down to where the liner will connect: the stove’s flue collar, the insert’s outlet, or the tee for a stove that connects through a thimble.
2. For a fireplace insert, measure down to the damper area, then add the distance from there to the insert outlet once it is in place.
3. Add about 12″ for the top plate, the termination and trimming. It is easy to cut a liner shorter; you cannot make it longer.
4. Round up to the next available kit length (commonly 15, 20, 25, 30 or 35 ft).

Flexible or rigid?

For wood, oil and gas, 316Ti or 316L/Ti stainless is the standard choice. An example is our 316L/Ti stainless steel chimney liner.

Kit checklist: everything the liner needs

Buying a chimney liner kit is the easiest way to get matched parts. Make sure your system includes:

• Liner in the correct diameter and length.
• Top plate to seal the top of the flue tile and support the liner.
• Rain cap / termination. Many kits include one; a full-width chimney cap also protects the crown.
• Bottom connection: a stove adapter or appliance connector for inserts, or a tee and tee cap for stoves connecting through a thimble.
• Insulation where required. Many liner listings and appliance manuals require insulation when the masonry does not meet clearance requirements, and it also improves draft.
• Clamps and hardware: hose clamps, termination adapters, and a pulling cone or nose cone for installation.

Gas inserts often use a manufacturer-specific co-axial liner kit instead. For example, see the Majestic direct vent insert liner kits, or the ExtraFlex insert liner kit for wood inserts.

Frequently asked questions

Can I use a bigger liner than my stove’s outlet?

Generally you should match the appliance outlet. An oversized flue can cool the gases, reduce draft and increase creosote. Follow the appliance manual.

Do I need insulation?

Often, yes. Whether insulation is required depends on the liner’s listing, the chimney construction and clearances, and the appliance. Insulation also keeps flue gases hotter, improving draft and reducing creosote.

What if my chimney has two flues?

Each appliance needs its own flue. Measure each flue you plan to line separately, and use a multi-flue cap if needed.

How do I measure if I can’t get on the roof?

Ask a chimney professional to measure and inspect the flue. An inspection before relining is recommended anyway, to confirm the masonry and crown are sound.

Do you install liners?

No. We sell and ship liners and kits only. Have a qualified installer fit the liner following the manufacturer’s instructions and local code.

Send us your measurements and appliance model, and a specialist will help you pick the right kit: (540) 212-9374 (Mon–Fri 9–5 ET). Browse all chimney liners and liner kits.