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The heat index combines air temperature and humidity into a single "feels like" temperature that reflects how hot the human body actually perceives it.
This calculator applies the official U.S. National Weather Service Rothfusz regression to find the heat index from temperature and relative humidity.
How it works
Enter the air temperature in °F and relative humidity as a percentage. The calculator first checks a simplified formula — if the result averages below 80°F with the actual temperature, that simpler value is used directly. Otherwise, it applies the full Rothfusz regression, with additional corrections for very low or very high humidity.
- Enter air temperature (°F).
- Enter relative humidity (%).
- Click Calculate to see your results.
Examples
95°F at 60% humidity
At 95°F air temperature with 60% relative humidity, the heat index is about 113°F — noticeably hotter than the actual air temperature.
Who should use it
- Checking how dangerous the heat actually feels before outdoor work or exercise.
- Understanding heat advisory thresholds issued by local weather services.
Industry applications
- Meteorology and weather forecasting
- Outdoor work safety and athletics
Advantages
- Uses the official, standardized NWS Rothfusz regression, including low/high humidity corrections.
- Matches the heat index values reported in official weather forecasts.
Limitations
- Most accurate in the shade — direct sunlight can add substantially more to the perceived temperature.
Common mistakes to avoid
- Confusing heat index (a "feels like" figure incorporating humidity) with the actual measured air temperature.
- Entering humidity as a decimal (like 0.6) instead of a percentage (60).
Best practices
- Use heat index values, not raw temperature, to judge heat exhaustion and heat stroke risk during outdoor activity.
Tips
- A heat index of 103°F or higher is generally considered dangerous, with prolonged exposure or physical activity increasing the risk of heat-related illness.