Fireline Intensity (FLI) measures the rate of heat energy released per unit length of a fire's front, indicating the fire's severity at any given point along its perimeter. FLI is expressed in kilowatts per meter (kW/m) in metric systems and British Thermal Units per foot per second (BTU/ft/s) in the US system.
FLI is calculated as a function of three factors: the heat yield of the fuel (energy content per unit mass), the available fuel load (mass of fuel per unit area consumed in the flaming zone), and the Rate of Spread (ROS). Because FLI incorporates both the intensity and speed of combustion, it is a more complete measure of fire energy than ROS or Flame Length (FL) alone.
FLI values are often used to categorize suppression feasibility. These thresholds vary by fuel type and terrain and are best interpreted alongside other fire behavior outputs such as FL and Fire Behavior Index (FBI).