Heat index calculator
How hot it really feels from the temperature and humidity.
About this calculator
This heat index calculator shows how hot it really feels when humidity is added to the air temperature.
High humidity stops sweat from evaporating, so the body cools less well. The heat index and danger level help you plan outdoor work, sport and travel in hot weather.
How it works
Uses the US National Weather Service formula, a regression of temperature and humidity by Rothfusz, with the NWS adjustments for very dry and very humid air.
Last reviewed:
Worked example
- Temperature in: °F
- Air temperature: 92
- Relative humidity: 60%
Feels like: 105 °F (Danger)
Common questions
What is the heat index?
The temperature it feels like to the human body when humidity is combined with the air temperature, measured in the shade.
When is the heat index dangerous?
From about 32 °C (90 °F) take extra care, and from about 39 °C (103 °F) heat stroke becomes likely with long exposure or hard work.
How can I stay safe in extreme heat?
Drink water often, rest in the shade or indoors, avoid hard work in the hottest part of the day, and check on older people and children.
Sources and further reading
The story behind it
Where the idea came from: The heat index was developed by the researcher Robert Steadman in 1979, and the US weather service later adopted it.
Steadman worked out how clothing, sweat and humidity change the body's heat balance. His tables were later turned into the formula used here, and heat index warnings now help prevent heat illness every summer.
Why you can trust this calculator
- It runs in your browser. Nothing you type into it is sent anywhere.
- The method is explained on this page, so you can check it yourself.
- Free to use, with no sign-up.
- Results are estimates, so please double-check anything important.
- Spotted a mistake? Tell us and we'll look into it.
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