Heat Index Calculator
Find out how hot it feels with the humidity.
Heat plus humidity
When the air is humid, sweat evaporates more slowly, so your body cools less well and it feels hotter than the thermometer says. The heat index combines temperature and relative humidity into one number that shows how hot it feels.
Example
On the first tab (°f) with the starting values shown (Air temperature 90 °F; Relative humidity 65 %), Heat index comes out as 102.7. Other results include risk level: Extreme caution: heat cramps and exhaustion are possible.
The index assumes shade and light winds, and direct sun can add up to about 15 °F (8 °C). It is meant for temperatures above about 80 °F (27 °C). In hot weather, drink water, rest in the shade and check on people at risk.
Frequently asked questions
What is a dangerous heat index?
Above about 103 °F (39 °C) is dangerous for prolonged exposure, and above 125 °F (52 °C) is extremely dangerous.
Does humidity matter more than temperature?
Both matter. At high humidity, even moderately warm days can feel oppressive.
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