The Burning Index (BI) estimates the difficulty of containing a fire under near worst-case conditions. It is derived from a combination of the Spread Component, which rates how fast a headfire spreads, and the Energy Release Component, which rates the available energy at the flaming front. The scale is open-ended, with no fixed maximum. Dividing the value by 10 gives an approximate flame length in feet at the head of the fire, so a BI near 40 corresponds to a flame length of about 4 feet. Higher values indicate greater potential fire intensity and harder containment.
A computed BI represents the near upper limit expected across the rating area, meaning the value reflects what a fire could do in the worst fuel, weather, and topography conditions present, not average conditions.