Heat Index Calculator
Humidity makes heat feel far worse than the thermometer says. Enter the air temperature and relative humidity to get the NOAA "feels-like" heat index and your sun-exposure risk level.
Calculate your heat index
Based on the official NOAA / NWS Rothfusz regression equation. Try the example first.
Results
How is this calculated?
The NOAA Heat Index (Rothfusz regression) is the apparent temperature — what a person actually feels, combining air temperature T (°F) and relative humidity RH (%). The regression is only valid for T ≥ 80 °F and RH ≥ 40%; below those thresholds humidity barely changes comfort, so the actual air temperature is shown instead.
HI = -42.379 + 2.04901523·T + 10.14333127·RH - 0.22475541·T·RH - 0.00683783·T² - 0.05481717·RH² + 0.00122874·T²·RH + 0.00085282·T·RH² - 0.00000199·T²·RH²
Source: U.S. National Weather Service (NWS) / NOAA Heat Index calculator, Rothfusz (1990). Risk bands follow NWS heat advisory criteria. Planning estimate only — check local alerts.