Flashback Velocity Margin Calculator
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: July 26, 2026Flashback Velocity Margin Calculator
Why this matters: flashback - a flame traveling backward into a burner or premix tube instead of staying anchored at the tip - is a genuine safety hazard in gas appliances and industrial burners. It happens when the unburned gas is not flowing outward fast enough to outrun the flame's own burning velocity.
Formula: Margin = Vjet - SL
A positive margin means the flow is winning and the flame stays put at the burner tip. A margin at or below zero means the flame can travel upstream.
Example: a 2.0 m/s jet against a 0.4 m/s burning velocity gives a 1.6 m/s margin - a comfortable buffer. But that margin shrinks fast if burner turndown reduces jet velocity while burning velocity stays constant (or rises, as it does with fuel-air ratio changes or preheating).
How to Use This Calculator
- Enter the unburned gas jet velocity leaving the burner.
- Enter the mixture's laminar burning velocity.
This calculator flags the risk level directly in the result. For a related failure mode at the other extreme - the flame lifting off and blowing out entirely - flow velocity far exceeding burning velocity is the concern instead of flashback.
See also: Bunsen Burner Flame Cone Angle Calculator, which uses the same two velocities.