Chimney Size Calculator
Enter the width and height of the fireplace opening, the chimney height and the flue shape. The calculator gives the flue area, the liner size to use and the height the chimney should rise above the roof.
- Flue area
- Liner size
- Chimney height
For a wood stove, use the stove maker's flue size instead.
The flue should be at least a tenth of the fireplace opening. A 36 × 28 in opening is 1,008 sq in. A round flue needs 101 sq in, a 12 in liner. A square flue needs an eighth, 126 sq in, a 12 × 16 in nominal liner.
Chimney flue size chart
Chimney 15 ft or taller. Search for an opening.
| Opening (in) | Area (sq in) | Round flue (sq in) | Square flue (sq in) |
|---|---|---|---|
| 24 × 20 | 480 | 48 | 60 |
| 28 × 24 | 672 | 67 | 84 |
| 30 × 24 | 720 | 72 | 90 |
| 32 × 26 | 832 | 83 | 104 |
| 36 × 28 | 1008 | 101 | 126 |
| 40 × 30 | 1200 | 120 | 150 |
| 42 × 32 | 1344 | 134 | 168 |
| 48 × 32 | 1536 | 154 | 192 |
| 54 × 36 | 1944 | 194 | 243 |
Square inches.
Opening and flue
28 in tall opening. Slide the opening width.
The flue grows with the opening.
How to size a chimney flue
Opening to flue. Step through it.
Opening
Width × height.
A cap keeps rain and birds out of the flue.
Round, square or rectangular flue
Flue shapes. Compare them.
Best draw.
Common.
Wide openings.
Smoke spirals up a flue, so round flues work best for their size.
Before you build
Chimney checks. Tick each item.
Tick each item as you go.
Have the chimney swept every year it is used.
What you get
Flue sizing. Each tile explains one result.
Square inches.
Size.
Above the roof.
Area.
Used.
A flue that is too small lets smoke into the room.
Frequently Asked Questions
No question matches that search.
What size flue does my fireplace need?
At least a tenth of the opening for a round flue, or an eighth for a square one.
What size flue for a thirty six inch fireplace?
About a twelve inch round or sixteen by sixteen inch square liner.
How tall should a chimney be?
At least fifteen feet, and three feet above where it leaves the roof.
What happens if a flue is too small?
Smoke spills into the room.
What happens if a flue is too big?
It draws poorly and builds up more creosote.
Does a short chimney need a bigger flue?
Yes. Use about a sixth of the opening.
What size flue for a wood stove?
Follow the stove maker, often six inches.
Smoke goes up, heat stays in
Size the flue to the fireplace and it draws cleanly every time you light it.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.