Chimney Liner Length Calculator
Order the length, not the tape reading. Liner length is the measured flue height plus a little to terminate above the crown, rounded up to the next whole foot.
1 Enter your numbers
The liner runs the full chimney height plus a little to terminate above the crown. 21.5 ft + 1.0 ft, rounded up, is 23 ft of liner — and that length drives the material cost. Measure from the appliance or smoke chamber to the top.
Buy a liner too short and it will not reach; buy it long and you are cutting expensive stainless off the top on the roof. Getting the length right first is the cheapest step in the whole reline. The rule is simple: measure the flue height — from where the appliance or the smoke chamber connects up to the top of the flue — then add a little overhang so the liner can terminate and cap above the crown, and round up to a whole foot.
The rounding is deliberate. Liner comes in whole feet, and you never want to come up short at the top, so the tool always rounds a fraction of a foot up to the next full one. That is why a 21.5-foot flue with a 1-foot overhang orders as 23 feet, not 22.5: the half foot rounds up and the extra becomes trim you cut on install, not a shortfall you cannot fix.
Formula
Height plus overhang, rounded up to whole feet:
liner_length = ceil(chimney_height_ft + overhang_ft)
Measure the flue itself — a tape or plumb line from the appliance/smoke-chamber connection to the top — not the outside of the masonry, which can differ from the flue run.
Worked example
Drop a weighted line down the flue and it reads 22.75 ft from the smoke-chamber connection to the top, and you want 1 ft to terminate and cap above the crown:
- Raw: 22.75 + 1 = 23.75 ft
- Rounded up: 24 ft of liner to order
That 24-foot number is what drives the material cost in the liner-cost and stainless-liner tools — feed it straight in.
Measure it right the first time
Measure the flue, not the chimney silhouette. The safest reading is a plumb bob or weighted tape lowered down the flue from the top, marked where it meets the appliance or smoke-chamber connection. On an offset chimney the liner follows the bends, so the flexible-liner run can be a touch longer than a straight drop — when in doubt, err long and trim.
Common mistakes: forgetting the overhang (the liner has to stick up to take a top plate and cap), and measuring the masonry height instead of the flue. Confirm the appliance connection point and the termination height against the manufacturer’s instructions and NFPA 211; the 3-2-10 height rule (a separate tool) tells you how tall the chimney itself must be.
Reference table
Height measured from the appliance or smoke chamber to the top of the flue, plus a little to terminate above the crown, rounded up to a whole foot. Order the length, not the exact tape reading.
| Measured height | Overhang | Order this length |
|---|---|---|
| 14.25 ft | 1.0 ft | 16 ft |
| 18.50 ft | 1.0 ft | 20 ft |
| 21.50 ft | 1.0 ft | 23 ft |
| 26.75 ft | 1.5 ft | 29 ft |
| 32.00 ft | 1.5 ft | 34 ft |