Chimney Flue Size Calculator
Flue area needed for a fireplace or stove opening.
Enter your values
Enter the fireplace opening width in in.
Enter the fireplace opening height in in.
Choose one of: 1:10 (chimney under 15 ft), 1:12 (chimney 15 ft+).
Enter the chimney height in ft.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Flue area needed for a fireplace or stove opening. |
|---|---|
| Category | Home & DIY |
| Inputs needed | Fireplace opening width, Fireplace opening height, Ratio rule, Chimney height |
| Main output | Required flue area |
| Formula | flue area = opening area / 10 |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Fireplace opening width(in)
- Enter the fireplace opening width in in.
- Fireplace opening height(in)
- Enter the fireplace opening height in in.
- Ratio rule
- Choose one of: 1:10 (chimney under 15 ft), 1:12 (chimney 15 ft+).
- Chimney height(ft)
- Enter the chimney height in ft.
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.
What do I need to enter into the Chimney Flue Size Calculator?
Just 4 values: fireplace opening width, fireplace opening height, ratio rule, chimney height. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Chimney Flue Size Calculator work out the 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. It applies the formula flue area = opening area / 10 and shows the working so you can check each step by hand.
Is the Chimney Flue Size Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.