Heat Index Calculator

About this calculator

This calculator finds the NWS heat index — the apparent temperature the National Weather Service uses, calculated with the Rothfusz regression from air temperature and relative humidity — since high humidity slows sweat evaporation and makes the air feel hotter than the thermometer says.

Meteorologists and weather services use it to issue heat advisories, and coaches, outdoor workers and event organizers use it to judge real heat risk and plan hydration breaks or rescheduling. Enter the temperature and humidity to see the heat index and its risk category.