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Heat Index Calculator
The heat index is the "feels like" temperature when humidity is combined with air temperature. It helps you judge heat-related risk.
Heat Index Calculator
Rothfusz regression — perceived "feels like" temperature
Rothfusz multi-variable regression formula (NWS standard)
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Heat Index (Feels Like)
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In °C
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In °F
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Hotter Than Actual
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About the Heat Index Calculator
The heat index is the "feels like" temperature when humidity is combined with air temperature. It helps you judge heat-related risk.
How to use the Heat Index Calculator
- Enter the Air Temperature and choose °F or °C.
- Enter the Relative Humidity (%).
- Press Calculate.
Formula and method
The US National Weather Service regression is used, valid for temperatures of about 80 °F (27 °C) or higher and humidity of 40% or more.
Worked example
At 95 °F and 60% humidity the heat index is about 113 °F.
When to use the Heat Index Calculator
- Finding how hot it feels when heat and humidity combine.
- Deciding whether it is safe to exercise or work outside.
- Planning outdoor events in summer.
Common mistakes to avoid
- Ignoring the heat index in direct sun. Being in full sun can make it feel much hotter.
- Using the formula in cool weather. It is designed for warm conditions.
Tips
- Heat index values of 103 °F or more call for caution, and much higher values can be dangerous.
- Direct sunlight can add up to 15 °F to the heat index.
- Drink water, take breaks in the shade and check on vulnerable people.
Frequently asked questions
Why does humidity make it feel hotter?
Sweat evaporates more slowly in humid air, so your body cools less effectively.
At what heat index is it dangerous?
Above about 40 °C (103 °F) heat illness becomes likely with prolonged exposure. Drink water and rest in the shade.