Enter the air temperature and the wet bulb temperature into the calculator to determine the Heat Load Index. This calculator can also evaluate any of the variables given the others are known.

Heat Load Index Formula

The following formula is used to calculate the Heat Load Index (HLI).

HLI = 0.5 * ((T + W) - 0.31 * (T - 14.4))


  • HLI is the Heat Load Index (°C)
  • T is the air temperature (°C)
  • W is the wet bulb temperature (°C)

To calculate the Heat Load Index, add the air temperature and the wet bulb temperature. Multiply the result by 0.5. Subtract 14.4 from the air temperature, then multiply the result by 0.31. Subtract this result from the first calculation to get the Heat Load Index.

How to Calculate Heat Load Index?

The following steps outline how to calculate the Heat Load Index (HLI).

  1. First, determine the air temperature (T) in degrees Celsius.
  2. Next, determine the wet bulb temperature (W) in degrees Celsius.
  3. Next, use the formula: HLI = 0.5 * ((T + W) – 0.31 * (T – 14.4)).
  4. Finally, calculate the Heat Load Index (HLI).
  5. After inserting the variables and calculating the result, check your answer with the calculator above.

Example Problem:

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

air temperature (T) = 25°C

wet bulb temperature (W) = 20°C