Heat Index Calculator
Calculate the heat index (feels-like temperature) with this free online calculator. Enter temperature and humidity to see the NWS heat danger level.
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How to use Heat Index Calculator
- 1Choose Fahrenheit or Celsius
- 2Enter the air temperature and relative humidity
- 3Read the heat index and its National Weather Service danger level
Frequently asked questions
What is the heat index?▾
The heat index is how hot it feels when humidity is combined with air temperature. High humidity slows sweat evaporation, so the body cools less and it feels hotter.
How is the heat index calculated?▾
This calculator uses the US National Weather Service Rothfusz regression, with its adjustments for very low and very high humidity. For example, 92 °F at 60% humidity feels like about 105 °F.
What heat index is dangerous?▾
The NWS uses Caution at 80–90 °F, Extreme Caution at 90–103 °F, Danger at 103–124 °F and Extreme Danger at 125 °F and above. Values are for shade; full sun can add up to 15 °F.
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