Summer Heat Index Calculator

About this calculator

The heat index, sometimes called the “feels-like” temperature, estimates how hot the air actually feels to the human body when high humidity is factored in alongside air temperature, because humidity slows the evaporation of sweat that the body relies on to cool itself. The US National Weather Service computes it using the Rothfusz regression, a multi-term polynomial fit (with adjustment terms for very low or very high humidity and temperature ranges) built to approximate Steadman’s 1979 apparent-temperature model.

The NWS groups the result into risk categories — Caution, Extreme Caution, Danger and Extreme Danger — each tied to the likelihood of heat-related illness such as heat cramps, heat exhaustion or heat stroke with prolonged exposure or physical activity, and forecasters use these thresholds to issue heat advisories and excessive-heat warnings. Enter the air temperature in degrees Fahrenheit and the relative humidity as a percentage to get the heat index and its risk category.