Chimney Flue Size Calculator
Chimney flue size from fireplace opening per IRC ratios, with stock liner picks.
What it does
A chimney flue must swallow the fireplace exhaust without choking it, and code sizes that relationship by shape: round flues run tighter than square ones because round sections draft better per inch. Guess small and smoke spills into the room; guess large and the flue runs cold.
The tool above takes the fireplace opening with the flue shape. It returns the opening area, the required net flue area, and the smallest stock liner that clears it.
The default 36 by 28 in opening needs 100.8 sq in net as a square flue, met by 12x12 tile at net 102. The metric mirror of 91.44 by 71.12 cm needs 650.3 sq cm with the same pick.
Ratios follow the International Residential Code with NFPA 211 behind it, checked against two state adoptions of the same language. All three agree on the 12, 10, and 8 divisors used here.
How to use
- Pick inches or centimeters.
- Enter the fireplace opening width.
- Enter the opening height.
- Pick the flue shape.
- Read the required net area first.
- Buy the smallest stock liner that clears it.
Formula
- opening
- Fireplace front opening area, sq in.
- divisor
- Shape divisor: 12 round, 10 square, 8 tall rectangular.
- net area
- Liner inside area from maker tables.
- aspect
- Longer liner side over shorter side.
Opening is front width times height in square inches; metric converts first (divide cm by 2.54). Aspect uses actual liner sides, longer over shorter. Always round liner selection up to net area at least the demand.
Worked example
Example: 36 by 28 in fireplace
Imperial defaults: 36 by 28 in opening, square flue.
- Opening: 36 in x 28 in = 1,008 sq in.
- Rule: square flue needs opening / 10.
- Required: 1,008 / 10 = 100.8 sq in net.
- Stock nets: 12x12 tile 102, 13x18 tile 173.
- Pick: 12x12, since 102 clears 100.8.
- Tool shows 1,008 sq in, 100.8 sq in net, 12x12 in tile (net 102).
Reference table
| Nominal | Net area | Covers square need |
|---|---|---|
| 12x12 tile | 102 | Up to 102 |
| 8.5x18 tile | 101 | Rectangular only |
| 13x18 tile | 173 | Up to 173 |
| 12 in round | 113 | Round up to 113 |
| 10 in round | 78 | Round up to 78 |
Limits
- Ratio method only; short stacks need the height figure method.
- One appliance per flue unless engineered.
- Throat and smoke chamber must match flue area.
- Local code and listings override this page.
Common mistakes
- Sizing from nominal outside dimensions.
- Ignoring the 2:1 rectangular aspect jump.
- Oversizing into a cold, creosote flue.
- Sharing one flue between two stoves.
- Stopping the flue short of termination height.
Try it yourself
Quick checks. Answers sit tucked under each question.
Default opening as tall rectangular?
Need 1,008 / 8 = 126 sq in: 8.5x18 (101) fails, 13x18 (173) passes.
30 by 24 in opening, round flue?
Opening 720, need 720 / 12 = 60 sq in: 10 in round (78) passes.
24 by 24 in opening, square?
Opening 576, need 57.6 sq in: 12x12 tile (102) passes.
Questions
What are the flue area ratios?
Round needs opening / 12, square and low ratio rectangular need / 10, tall rectangular 2:1 or more needs / 8.
Net or nominal area?
Net inside area from the liner tables, not the nominal outside size. A 12x12 tile nets 102 sq in.
How is chimney height measured?
From the firebox floor to the flue top. Short stacks may need the figure method instead of ratios.
How high must the flue terminate?
Yes, above the roof line: 3 ft past penetration and 2 ft past anything within 10 ft.
What fails with an oversize flue?
Weak draft, smoke spillage, and creosote from cool gas. Match the ratio and confirm draft with a sweep.
Can two stoves share one flue?
One appliance per flue unless the code sizes a shared flue for the combined load.