Flue-Liner Size Calculator
Size a liner to the appliance collar, not the old masonry flue — enter the outlet diameter and get the next standard liner up.
1 Enter your numbers
Size a liner to the appliance, not the old masonry flue — match the manufacturer’s outlet (usually 6" for a modern wood stove). Your 6.0" outlet is 28.3 in², so a 6" round liner matches it. A liner smaller than the outlet chokes it, and a much larger flue runs cool and forms creosote.
When you put a modern appliance on an old chimney, the liner size comes from the appliance collar — full stop. A stove rated for a 6-inch flue wants a 6-inch liner. The huge masonry flue behind the wall is irrelevant except that it is usually the problem, not the answer.
Enter the outlet diameter and the tool gives the outlet cross-section and the standard liner to buy. Go smaller than the collar and you choke the appliance — sluggish, smoky, dangerous. Go a lot bigger and the flue gases lose heat, slow down, and drop creosote on the walls. Matching the collar is the rule the manufacturer’s manual and NFPA 211 both point to.
Formula
Outlet-driven, then rounded to what you can buy:
- Outlet cross-section
A = π × (d ÷ 2)² - Liner cross-section
≥ A(never smaller than the outlet) - Liner diameter = the appliance outlet, rounded up to the next standard size
Same circle-area identity as the rest of the flue math; the constraint is simply “at least the outlet.”
Worked example
A high-output stove with a 7-inch collar: the outlet area is π × 3.5² = 38.5 in². The liner has to carry at least that, so you run a 7-inch liner — not a 6″ because that is what was on the shelf, and not an 8″ because the masonry flue is wide. Reducing a 7″ stove to a 6″ liner strangles it and voids the listing; oversizing it to match the old 10×10 tile leaves the gases cold and creosote-prone.
Size to the collar, confirm with the manual
Read the collar, read the manual. The single number you need is stamped near the flue outlet or printed in the install manual as the required flue diameter. Field reminders:
- Do not average the appliance outlet and the masonry flue — size to the appliance.
- An insulated liner matched to the outlet beats a bare oversized one for keeping gases hot.
- Category and fuel matter: a gas Category I appliance follows its own listing, not this shortcut.
Reference table
Standard round liner diameters and the cross-section each one carries. Match a liner to the appliance outlet (below), never to the old masonry flue.
| Round liner | Cross-section |
|---|---|
| 4″ round | 12.6 in² |
| 5″ round | 19.6 in² |
| 6″ round | 28.3 in² |
| 7″ round | 38.5 in² |
| 8″ round | 50.3 in² |
| 10″ round | 78.5 in² |
| 12″ round | 113.1 in² |
| 14″ round | 153.9 in² |
| 15″ round | 176.7 in² |
| 18″ round | 254.5 in² |
Liner size by appliance — labeled typicals, confirm the manufacturer’s outlet:
| Appliance / outlet | Typical outlet | Recommended liner |
|---|---|---|
| Modern wood stove / insert | 6″ | 6″ round |
| Larger wood stove / high output | 8″ | 8″ round |
| Wood-fireplace flue (by opening) | per listing | per the 1/10 rule / manufacturer |
| Gas appliance (Cat. I) | per listing | per the 1/10 rule / manufacturer |
Frequently asked questions
What size liner does a wood stove need?
Almost always the size of its flue collar — 6″ for most modern stoves, 7–8″ for larger high-output units. Match the collar and confirm with the stove’s manual; do not size to the old chimney.
Can I put a 6-inch stove into an 8-inch liner?
You can, but you usually should not. The extra area lets the gases cool, weakening draft and building creosote. If a larger liner is unavoidable, insulate it to keep the flue hot.
Why size to the appliance instead of the chimney?
The appliance is engineered around a specific flue size for a specific draft. The masonry flue is just the shell; relining it down to the collar is what makes the pairing work.
Does a gas appliance follow the same rule?
No — a gas Category I appliance is vented per its listing and the vent-sizing tables, which account for input, height and connector length. Use the manufacturer’s data, not this wood-appliance shortcut.