Wood-Stove Flue Size Calculator
Match the stove collar and catch the oversized masonry flue that runs your gases cold — the two things that make or break a wood-stove install.
1 Enter your numbers
A modern wood stove wants a flue matched to its collar — usually 6". Your existing 68 in² masonry flue is more than twice the outlet, so the gases run cool and slow — an insulated liner sized to the outlet is the fix. Confirm with the stove’s manual and NFPA 211.
A wood stove has two flue sins, and this tool checks both. First, the liner has to match the collar — a 6-inch stove wants a 6-inch flue. Second, and the one that surprises people: venting that stove into a big open masonry flue is worse than helpful. All that extra volume lets the exhaust cool and slow, so it drops creosote and drafts lazily.
Enter the stove outlet and the clear area of the flue you would connect to. If the existing flue is more than about twice the outlet, the tool flags it for an insulated liner sized to the collar — the standard fix that keeps the gases hot all the way up. If there is no chimney (a new class-A run), just size to the collar.
Formula
Match, then check the ratio:
- Outlet area
A = π × (d ÷ 2)² - Liner
≥ A, sized to the collar and rounded up to a standard size - Reline flag if
existing flue area > 2 × A— oversized, gases run cool
The 2× threshold is a labeled field rule of thumb, not a code line — it catches the classic “stove into a fireplace flue” mismatch.
Worked example
Put an 8-inch stove on a chimney whose masonry flue clears about 130 in² (a big nominal 13×13 tile). The outlet area is π × 4² = 50.3 in². Two times that is 100.5 in², and 130 sails past it — so this flue is oversized. The answer is not “good, plenty of room”; it is an 8-inch insulated liner dropped down the flue so the stove sees a right-sized, hot passage instead of a cold cavern.
Match the collar, reline an oversized flue
Get the clear flue area, not the tile label. Measure the inside of the flue tile or take it from the clay-tile typicals, and compare to the collar. From the field:
- “It fits” is not “it is sized.” A stove rattling around inside a fireplace flue needs a liner.
- Insulate the liner — a bare stainless liner in a cold exterior chimney still loses heat.
- A short connector run and a straight liner draft best; every elbow costs you.
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
Can I vent a wood stove into my fireplace chimney?
Only through a properly sized liner. The open fireplace flue is far too large for a stove, so the gases cool and creosote builds. Run an insulated liner sized to the stove collar down the existing flue.
How do I know if my flue is too big for the stove?
Compare the clear flue area to the stove outlet area. If the flue is more than roughly twice the outlet, it is oversized for that stove and should be relined to the collar size.
Why insulate the liner?
A hot flue drafts well and stays cleaner. Insulation around the liner keeps the exhaust from dumping its heat into a cold masonry chimney, which is exactly what causes weak draft and creosote.
Do I need a liner for a brand-new stove with class-A pipe?
No — a new class-A chimney run is already sized to the stove. The liner question only comes up when you are connecting to an existing, usually oversized, masonry flue.