Heat Index Calculator — Calculate the "Feels Like" Temperature

Air Temperature (°F)Formula is most accurate at 80°F or aboveEnter a valid temperature (80°F or above).

Relative Humidity (%)Enter a valid humidity (0-100%).

Heat Index

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How to Use This Tool

Enter the current air temperature in °F (80°F or above for best accuracy).
Enter the current relative humidity percentage.
Click Calculate for the heat index "feels like" temperature.

The Rothfusz Regression Formula

The heat index uses a complex regression equation (the Rothfusz regression, adopted by the National Weather Service) combining temperature and relative humidity to estimate the perceived temperature — this formula is most accurate at temperatures of 80°F and above.

Why Humidity Makes Heat Feel Worse

Your body cools itself primarily through sweat evaporation. High humidity slows this evaporation, reducing your body's ability to cool itself — meaning humid heat feels significantly hotter and is more physically taxing than dry heat at the same actual temperature.

Heat Index Danger Categories

80-90°F: Caution — fatigue possible with prolonged exposure
90-103°F: Extreme caution — heat cramps/exhaustion possible
103-124°F: Danger — heat exhaustion likely, heat stroke possible
125°F+: Extreme danger — heat stroke highly likely

These are general NWS reference categories — individual heat tolerance varies based on health, age, activity level, and acclimatization.

Frequently Asked Questions

Why does humidity matter so much for how hot it feels?

Your body relies on sweat evaporation to cool down — high humidity slows or prevents that evaporation, reducing your body's cooling ability and making the same air temperature feel significantly hotter and more dangerous.

Is the heat index formula accurate at all temperatures?

No, it's specifically calibrated for and most accurate at temperatures of 80°F and above with a heat index calculation — it isn't meaningful or reliable for cooler conditions.

This calculator is provided for general informational and estimation purposes only. Results should not be treated as professional financial, tax, legal, medical, or engineering advice. Always verify critical calculations with a qualified professional or official source before making decisions.