Heat Index Calculator

Estimate how hot it really feels by combining actual air temperature and humidity. The heat index helps evaluate discomfort and heat-related health risks.

What Is Heat Index?

Heat Index (also known as “feels like” temperature) is a measurement that combines air temperature and humidity to show how hot it actually feels to the human body.

Heat Index Formula

The NOAA formula (applied when temperature ≥ 80°F and humidity ≥ 40%):

HI = -42.379 + 2.04901523T + 10.14333127R 
     - 0.22475541TR - 0.00683783T² 
     - 0.05481717R² + 0.00122874T²R 
     + 0.00085282TR² - 0.00000199T²R²

Heat Index Categories

  • 80–90°F (27–32°C): Caution — fatigue possible
  • 90–105°F (32–41°C): Extreme caution — heat cramps likely
  • 105–130°F (41–54°C): Danger — heat exhaustion likely
  • 130°F+ (54°C+): Extreme danger — heat stroke imminent

Tips for Staying Safe

  • Stay hydrated
  • Avoid heavy outdoor activity
  • Wear loose, light clothing
  • Take frequent shade or AC breaks

Frequently Asked Questions

High humidity reduces the body's ability to cool through sweat evaporation.
Heat index is an estimate but closely matches perceived temperature under shaded conditions.
Wind does not affect heat index; it influences wind chill instead.