Enter the Initial Spread Index and Buildup Index into the calculator to determine the Fire Weather Index. This calculator can also evaluate any of the variables given the others are known.

Fire Weather Index Formula

The following formula is used to calculate the Fire Weather Index (FWI). 

FWI = 0.434 * ln(ISI) + 0.537 * ln(BUI) - 1.233

Variables:

  • FWI is the Fire Weather Index
  • ISI is the Initial Spread Index
  • BUI is the Buildup Index ln is the natural logarithm function

To calculate the Fire Weather Index, take the natural logarithm of the Initial Spread Index and multiply it by 0.434. Then, take the natural logarithm of the Buildup Index and multiply it by 0.537. Add these two results together and subtract 1.233 to get the Fire Weather Index.

What is a Fire Weather Index?

A Fire Weather Index (FWI) is a meteorological tool used to estimate the risk or danger of wildfires in a specific area. It takes into account various factors such as temperature, humidity, wind speed, and recent rainfall to calculate the potential intensity of a wildfire. The index is used by fire management agencies to plan and allocate resources, and to issue warnings to the public when the risk is high.

How to Calculate Fire Weather Index?

The following steps outline how to calculate the Fire Weather Index (FWI) using the given formula:

  1. First, determine the value of the Initial Spread Index (ISI).
  2. Next, determine the value of the Buildup Index (BUI).
  3. Using the formula FWI = 0.434 * ln(ISI) + 0.537 * ln(BUI) – 1.233, substitute the values of ISI and BUI into the formula.
  4. Calculate the natural logarithm (ln) of ISI and BUI.
  5. Multiply the natural logarithm of ISI by 0.434.
  6. Multiply the natural logarithm of BUI by 0.537.
  7. Subtract 1.233 from the sum of the previous two calculations.
  8. The result is the Fire Weather Index (FWI).

Example Problem:

Use the following variables as an example problem to test your knowledge:

ISI = 10

BUI = 20