Chimney Flue Size Calculator
Flue area needed for a fireplace or stove opening.
Enter your values
ⓘ Live results
Results
- Required flue area
- 104.4 sq in
- Round liner diameter
- 11.5 in (use 12 in)
- Fireplace opening
- 1,044 sq in
- Chimney height
- 18 ft (good draft)
Working
- 11,044 sq in / 10 = 104.4 sq in of flue
Quick answer
A masonry fireplace flue should be about 1/10 of the fireplace opening area (1/12 for tall chimneys), and never smaller than the appliance outlet.
flue area = opening area / 10
How to use the Chimney Flue Size Calculator
- 1Measure the fireplace opening width and height.
- 2Use 1:10 for chimneys under 15 ft and 1:12 for taller stacks.
- 3Never size a liner smaller than the appliance manufacturer's outlet.
Worked example
Using the values the calculator loads with:
Inputs
- Fireplace opening width36 in
- Fireplace opening height29 in
- Ratio rule1:10 (chimney under 15 ft)
- Chimney height18 ft
Results
- Required flue area104.4 sq in
- Round liner diameter11.5 in (use 12 in)
- Fireplace opening1,044 sq in
- Chimney height18 ft (good draft)
- 1,044 sq in / 10 = 104.4 sq in of flue
Frequently asked questions
What happens if the flue is too big?
Flue gases cool and slow down, draft weakens, and you get smoke spillage plus heavy creosote build-up.
Does chimney height matter?
Yes. Draft is driven by stack height and temperature difference; a chimney should extend at least 3 ft above the roof and 2 ft above anything within 10 ft.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.