Heat Index Calculator
"Feels Like" Hot Weather Temperature
Heat Index (Feels Like)
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Ready for Calculation
Enter temperature and relative humidity to calculate the heat index.
Understanding the Heat Index
The heat index combines air temperature and relative humidity to estimate how hot it actually feels to the human body. High humidity slows the evaporation of sweat, your body's primary cooling mechanism, making the air feel far hotter than the thermometer alone suggests.
Humidity Blocks Evaporative Cooling
Sweat cools you as it evaporates. In humid air, the surrounding air is already saturated with moisture, so sweat evaporates more slowly and cools you less effectively.
Heat Illness Risk
Heat index values above 103°F (39°C) fall into the "danger" category for heat cramps, heat exhaustion, and heat stroke with prolonged exposure or physical activity.
The Rothfusz Regression
This calculator uses the same multi-variable regression the National Weather Service uses operationally, valid for temperatures at or above 80°F (26.7°C) with humidity at or above 40%.
Opposite of Wind Chill
Heat index measures how hot you feel in summer; our Wind Chill Calculator measures how cold you feel in winter wind.
Key Takeaways
- Shade Matters: Heat index values assume shaded conditions — full direct sunlight can add 15°F (8°C) or more to the perceived temperature.
- Stay Hydrated: High heat index conditions significantly increase fluid loss — increase water intake well before you feel thirsty.
- Valid Range Only: Below 80°F (26.7°C), the heat index formula isn't meaningful and this calculator will flag it.