Heat index is a specific measurement of how hot it actually feels to the human body when air temperature is combined with relative humidity
The heat index — often called the "feels-like" temperature — measures how hot the air actually feels to the human body when relative humidity is factored in alongside the air temperature. When humidity is high, sweat evaporates more slowly, which reduces the body's ability to cool itself. That's why a humid 90°F day can feel far more dangerous than a dry 90°F day, even though the thermometer reads the same.
This calculator uses the same regression formula developed by the U.S. National Weather Service (NWS), commonly known as the Rothfusz formula. It combines live air temperature and relative humidity for your selected city to produce a single apparent-temperature value. Select your state and city above and press Calculate Heat Index to see live results.
The National Weather Service groups heat index values into four risk categories:
| Category | Heat Index Range | Effect on the Body |
|---|---|---|
| Caution | 80°F – 90°F | Fatigue possible with prolonged exposure or activity |
| Extreme Caution | 90°F – 103°F | Heat cramps and heat exhaustion possible |
| Danger | 103°F – 124°F | Heat cramps and heat exhaustion likely; heat stroke possible |
| Extreme Danger | 125°F and higher | Heat stroke highly likely with continued exposure |
Outdoor workers, athletes and coaches, older adults, young children, and anyone with heart or respiratory conditions face elevated risk as heat index values climb. Many employers and school athletic programs use heat index thresholds to decide when to modify or cancel outdoor activity — checking this calculator before extended outdoor time is a simple way to plan ahead.
Planning for winter instead? Try the Snow Day Calculator, check your area's Heat Wave Probability for the season, or browse the Weather Blog for more guides.
The heat index is a measure of how hot it actually feels to the human body when relative humidity is combined with air temperature. It is not the same as the thermometer reading.
It's calculated using the Rothfusz regression formula developed by the National Weather Service, which combines temperature and relative humidity into a single apparent-temperature value.
Values from 90–103°F fall under Extreme Caution, 103–124°F under Danger, and 125°F+ under Extreme Danger, per NWS guidelines. See the full table above for details.
No — the heat index is the "feels-like" apparent temperature. Two locations with the same thermometer reading can have very different heat index values depending on humidity.
Outdoor workers, athletes, older adults, young children, and people with heart or respiratory conditions face higher risk at elevated heat index levels.