Heat Index Calculator
Find how hot it feels when humidity is added to the air temperature, using the US National Weather Service formula.
About the Heat Index Calculator
Humid air slows the evaporation of sweat, so the body feels hotter than the thermometer shows. The NWS computes the heat index with the Rothfusz regression, plus corrections for very dry and very humid conditions, and a simpler Steadman formula when the result is below 80 °F.
The formula
HI = −42.379 + 2.049 T + 10.143 RH − 0.2248 T·RH − … (T in °F)
Worked example
Example: Feels like = 58.4 °C (137.2 °F)
- Inputs: Temperature in: °F; Air temperature: 100; Relative humidity: 66%.
- Formula: HI = −42.379 + 2.049 T + 10.143 RH − 0.2248 T·RH − … (T in °F)
- Result: Feels like = 58.4 °C (137.2 °F)
Questions people ask
Is heat index the same as temperature?
No, it's how hot it feels in the shade.
How do I stay safe?
Drink water, rest in shade, avoid strenuous activity in the hottest hours and check on vulnerable people.