🥵 Heat Index Calculator
Estimate how hot it actually feels outside by combining air temperature and humidity into a single heat index value.
What heat index measures
Heat index combines air temperature and relative humidity into a single ‘feels like’ number. High humidity slows the evaporation of sweat, which reduces the body’s ability to cool itself, so hot and humid air feels hotter than the thermometer reading alone suggests.
The formula
For temperatures at or above 80°F, this calculator uses the U.S. National Weather Service’s Rothfusz regression, a polynomial fit to more detailed heat-stress models, with standard low-humidity and high-humidity correction terms. For temperatures below 80°F, it uses a simpler averaging formula since the full regression is not designed for cooler conditions.
Heat index values are general estimates for shaded, light-wind conditions; direct sun exposure can add significantly to the perceived heat. This tool is for general educational and planning purposes, not official weather guidance.