Chimney Liner Size Calculator — What Size Flue Liner Do I Need?
A chimney liner size calculator that sizes your flue the right way: enter the inside width and height of your rectangular flue tile and get its cross-sectional area, the minimum round liner area per the NFPA 211 1:1 rule of thumb, and the smallest standard round liner diameter that meets it. Instant results, no sign-up.
Enter your flue size
Measure the inside of the flue tile at the top of the chimney, edge to edge.
Your results
How to use this calculator
Measure the inside of your rectangular flue tile at the top of the chimney — inside edge to inside edge, in inches — and type the width and height into the calculator. The default 8 × 12 in matches one of the most common flue tile sizes. Results update instantly as you type.
The result tells you three things: your flue's cross-sectional area, the minimum round liner area the NFPA 211 1:1 rule of thumb calls for, and the smallest standard round liner diameter that meets it. Take these numbers to your liner supplier or chimney sweep — and if the flue tile is cracked, spalling, or missing pieces, mention it, since that affects the relining plan.
How it works
The math is straightforward geometry. For a rectangular flue, the cross-sectional area is width × height — an 8 × 12 in flue is 8 × 12 = 96 sq in. For a round liner, the area is π × (diameter ÷ 2)² — an 11 in liner is π × 5.5² ≈ 95.0 sq in.
The sizing rule comes from NFPA 211's rule of thumb: for most appliances, a round liner's cross-sectional area should be at least equal to the flue's interior area (1:1). The calculator compares your flue area against the areas of standard round liner diameters — 5.5, 6, 7, 8, 9, 10, 11, and 12 inches — and recommends the smallest standard size whose area meets (or is within about 2% of) your flue area. For 96 sq in, that's an 11 in liner (≈ 95.0 sq in); a 12 in liner (≈ 113.1 sq in) is the next step up if you want extra draft margin.
Important: this is an estimate and a rule of thumb, not a code certification. Real-world sizing also depends on your appliance, flue height, and local code. Have a certified chimney professional inspect the chimney (an NFPA 211 Level II inspection is the norm when installing a new liner) before you buy.
Frequently asked questions
What size chimney liner do I need for an 8x12 flue?
An 8x12 flue has a cross-sectional area of 96 square inches. The smallest standard round liner that covers that area is an 11 inch liner (about 95.0 square inches), following the NFPA 211 rule of thumb that round liner area should at least equal flue area. A 12 inch liner (about 113 square inches) gives extra margin. Have a certified chimney professional confirm the final size before buying.
What is the 1:1 rule for chimney liner sizing?
The 1:1 rule, drawn from NFPA 211, says a round chimney liner's cross-sectional area should be at least as large as the flue's interior area. An 8x12 inch rectangular flue (96 square inches) needs a round liner of about 96 square inches or more - roughly an 11 inch diameter liner. Undersized liners can cause poor draft, smoke spillage into the home, and faster creosote buildup.
Can I use a smaller chimney liner than my flue opening?
A liner smaller than the 1:1 area guideline is generally not recommended. An undersized liner weakens draft, which can push smoke and carbon monoxide back into the home and accelerate creosote buildup - a chimney fire risk. If your flue is damaged or oversized, a chimney professional can advise on relining options, but the liner should still meet the minimum area rule for your appliance.
How do I measure my chimney flue?
Measure the inside dimensions of the flue opening at the top of the chimney, from inside edge to inside edge, in inches - width by height for a rectangular flue tile. Measure before buying a liner, and check that the flue is clear of obstructions. Note that flue tiles are often nominal sizes (an 8x12 tile is close to those dimensions), so always measure the actual opening.
What are standard round chimney liner sizes?
Standard round stainless steel chimney liner diameters are 5.5, 6, 7, 8, 9, 10, 11, and 12 inches. Their cross-sectional areas run from about 23.8 square inches (5.5 inch) to about 113.1 square inches (12 inch). The calculator below picks the smallest standard size whose area meets the minimum area for your flue.