Flue Draft Calculator

Dial flue draft for safer garden fire features. See stack effect, losses, and net pull. Export results to share with installers and inspectors today.

Calculator

Choose units, enter flue geometry and temperatures, then (optionally) include fittings and flow losses.

Switch units anytime; results update on submit.
Used only for a draft “target band.”
Loss estimates need flow rate and fittings counts.
If unsure: 29.92 inHg or 101.33 kPa.
Used to estimate straight friction length.
Approximate exhaust flow for loss estimate.
Fittings & Minor Losses
Counts help estimate bends and components that reduce draft.
Typical K-values are used automatically.
Use for caps, screens, transitions, etc.
Tip: If you don’t know flow rate, run stack-only first. Then add a reasonable flow estimate and fittings counts to see how net draft changes.
Reset View Example Table

Example Data Table

Example values show how height and temperature can increase draft, while elbows and high flow raise losses.

Scenario Height Diameter Gas Temp Outdoor Temp Flow Elbows 90° Net Draft (approx)
Short, mild day 10 ft 6 in 350 °F 70 °F 120 CFM 2 Lower to moderate
Taller, colder day 16 ft 6 in 450 °F 35 °F 150 CFM 2 Moderate to strong
High losses 16 ft 6 in 450 °F 35 °F 220 CFM 5 May fall below target
These labels are qualitative. Use the calculator output for numeric estimates.

Formula Used

1) Stack-effect draft (pressure gain)

The draft comes from the density difference between outdoor air and hot flue gases:

ΔP_stack = g · H · (ρ_out − ρ_in)

Density uses the ideal gas relation: ρ = P / (R · T).

2) Loss estimate (optional)

Straight friction and fittings are estimated using:

ΔP_loss = ( f·L/D + K_total ) · (ρ·v²/2)

Where v = Q/A, and f is from a standard approximation.

Net draft is computed as ΔP_net = ΔP_stack − ΔP_loss (when losses are enabled), then converted to inches of water column and mm H₂O for convenience.

How to Use This Calculator

  1. Select your unit system and appliance type.
  2. Enter flue height, diameter, indoor flue gas temperature, and outdoor temperature.
  3. Set barometric pressure (use the default if unknown).
  4. For a deeper estimate, add flow rate, horizontal run, and fittings counts.
  5. Click Calculate Draft to see results above the form.
  6. Use CSV or PDF export to share results with your team.
If you see “Reverse / Backdraft Risk,” stop and verify venting. Wind, caps, combustion air, and building pressure can dominate real-world behavior.

Related Calculators

Fire pit BTU calculatorArea heat load calculatorOutdoor temperature drop calculatorWind chill impact calculatorRadiant heat range calculatorConvection loss calculatorHeat output conversion calculatorPropane burn rate calculatorWood burn rate calculatorFire pit runtime calculator

Important Note: All the Calculators listed in this site are for educational purpose only and we do not guarentee the accuracy of results. Please do consult with other sources as well.