How to Calculate Heat Index Formula

By Michael Keenan; Updated April 24, 2017
A thermometer measures temperature.

The heat index is a measure of how hot the weather feels to the human body, taking into account both the temperature and the relative humidity level. When the relative humidity level is high, the temperature feels warmer to the human body. As a result, the body dehydrates more quickly. To calculate the heat index, you need to know the temperature and the relative humidity.

Use a thermometer to measure the air temperature in degrees Fahrenheit and call it F. For example, if the temperature was 96 degrees Fahrenheit, F would be 96.

Divide the relative humidity by 100 to convert it from a percentage to a decimal and call it H. For example, if the relative humidity was 70 percent, you would divide 70 by 100 to get 0.7.

Use the following heat index formula: HI= -42.379 + 2.04901523_F + 10.14333127_H - 0.22475541_F_H - 6.83783_10^-3_F^2 - 5.481717_10^-2_H^2+1.22874_10^-3_F^2_H + 8.5282_10^-4_F_H^2 - 1.99_10^-6_F^2*H^2. The carets (^) represent exponents. You can also use an online heat index calculator to simplify the calculations (see Resources). For example, if you had a temperature of 96 degrees and a relative humidity of 0.7, you would get a heat index of about 126 degrees Fahrenheit.


If the heat index exceeds 130 degrees Fahrenheit, the danger of heat strokes and sun strokes is extremely high, according to the National Weather Service.

About the Author

Mark Kennan is a writer based in the Kansas City area, specializing in personal finance and business topics. He has been writing since 2009 and has been published by "Quicken," "TurboTax," and "The Motley Fool."