Burning Index (BI) is a number used by the National Oceanic and Atmospheric Administration (NOAA) to describe the potential amount of effort needed to contain a single fire in a particular fuel type within a rating area. The National Fire Danger Rating System (NFDRS) uses a modified version of Bryam's equation for flame length – based on the Spread Component (SC) and the available energy (ERC) – to calculate flame length from which the Burning Index is computed.

The equation for flame length is listed below:

F L = j [ ( S C 60 ) ( 25 ( E R C ) ) ] 0.46 {\displaystyle F_{L}=j\left[\left({\frac {SC}{60}}\right)(25(ERC))\right]^{0.46}}

where:

j is a scaling factor, SC is the spread component, and ERC is the Energy Release Component.

Consequently, the equation for the Burning Index is:

B I = j 1   F L {\displaystyle BI=j_{1}\ F_{L}}

where j 1 {\displaystyle j_{1}} is the Burning Index scaling factor of (10/ft). Therefore, dividing the Burning Index by 10 produces a reasonable estimate of the flame length at the head of a fire. A unique Burning Index (BI) table is required for each fuel model.