Heat Index Calculator
Calculate the heat index — how hot it actually feels — from air temperature and relative humidity using the U.S. National Weather Service formula.
About the Heat Index Calculator
Heat index combines air temperature and humidity to estimate how hot it actually feels to the human body, since high humidity slows the evaporation of sweat that would otherwise cool you down.
Formula: Uses the NWS Rothfusz regression, a polynomial in temperature (T) and relative humidity (RH), with adjustments applied at very low humidity/high heat and very high humidity/moderate heat.
The National Weather Service issues heat advisories and warnings based on these same risk categories — "Danger" and "Extreme Danger" levels carry real risk of heat cramps, heat exhaustion or heat stroke with prolonged exposure or physical activity.