Fireplace Chimney Draft Calculator
Theoretical draft pressure for a masonry chimney.
Enter your values
Enter the chimney height in ft.
Enter the outdoor temperature in °F.
Enter the average flue gas temp in °F.
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.
- Theoretical draft
- 0.0046 in wc
- In Pascals
- 1.2
- Guidance
- Working fireplaces typically need 0.03-0.08 in wc to draw properly
- Status
- Marginal — check for a short flue, cold chimney or excess resistance
Quick answer
Chimney draft is produced by the density difference between hot flue gas and outside air, and increases with taller flues and a bigger temperature difference.
Draft (in wc) = 0.256 x H x (1/Tout - 1/Tflue), temperatures in Rankine
At a glance
| What it does | Theoretical draft pressure for a masonry chimney. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Chimney height, Outdoor temperature, Average flue gas temp |
| Main output | Theoretical draft |
| Formula | Draft (in wc) = 0.256 x H x (1/Tout - 1/Tflue), temperatures in Rankine |
| Cost | Free — no sign-up, no download |
How to use the Fireplace Chimney Draft Calculator
- 1Enter the chimney height above the fire.
- 2Enter the outdoor air temperature and typical flue gas temperature.
- 3Compare the result to the 0.03-0.08 in wc range typically needed for reliable draft.
Inputs explained
- Chimney height(ft)
- Enter the chimney height in ft.
- Outdoor temperature(°F)
- Enter the outdoor temperature in °F.
- Average flue gas temp(°F)
- Enter the average flue gas temp in °F.
Worked example
Using the values the calculator loads with:
Inputs
- Chimney height25 ft
- Outdoor temperature50 °F
- Average flue gas temp350 °F
Results
- Theoretical draft0.0046 in wc
- In Pascals1.2
- GuidanceWorking fireplaces typically need 0.03-0.08 in wc to draw properly
- StatusMarginal — check for a short flue, cold chimney or excess resistance
Frequently asked questions
Why does a cold chimney smoke at startup?
Cold flue liner reduces the temperature difference driving draft; pre-warming the flue with a rolled newspaper helps establish flow.
Does chimney height always help?
Yes for draft strength, but very tall chimneys also cool the flue gas more before exit, partially offsetting the gain.
What do I need to enter into the Fireplace Chimney Draft Calculator?
Just 3 values: chimney height, outdoor temperature, average flue gas temp. 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 Fireplace Chimney Draft Calculator work out the answer?
Chimney draft is produced by the density difference between hot flue gas and outside air, and increases with taller flues and a bigger temperature difference. It applies the formula Draft (in wc) = 0.256 x H x (1/Tout - 1/Tflue), temperatures in Rankine and shows the working so you can check each step by hand.
Is the Fireplace Chimney Draft 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.