Flame Length (FL) is the distance measured from the average flame tip to the middle of the flaming zone at the base of the fire. FL is calculated as an average value at the fire's head and is expressed in feet (ft) or meters (m). It is an indicator of fire intensity and one of the two primary factors in the Fire Behavior Index (FBI), alongside Rate of Spread (ROS).
FL is derived from Fireline Intensity (FLI). Higher fuel loads, lower fuel moisture, and stronger winds all tend to produce longer flames.
Flame length is widely used to communicate suppression difficulty and crew safety boundaries. At lower lengths, fire can generally be attacked at the head by ground crews using hand tools. High flame lengths indicate extreme fire behavior in which only indirect and long-range strategies apply.
For utility customers, elevated FL values near infrastructure increase the risk of direct structure ignition and the likelihood that responding crews face conditions that preclude safe access.