Weather & Climate
Heat index calculator.
Estimate apparent temperature from air temperature and relative humidity using the National Weather Service’s published method.
Inputs
Apparent temperature
Fill the form and press Calculate.
How it works
This calculator first uses the NWS simple approximation. When that estimate reaches 80°F, it applies the Rothfusz regression and the NWS low- and high-humidity adjustments. The published equations are defined in Fahrenheit, so metric inputs are converted before calculation and converted back for display.
Source and limits
Source: National Weather Service heat index guidance and the NWS Rothfusz technical attachment. Heat index is an apparent-temperature estimate, not a body-temperature measurement. The regression is not intended for extreme conditions outside the range of the underlying studies.
Estimate only
Use local official heat alerts and occupational or medical guidance for safety decisions. Humidity, sun, wind, clothing, exertion, and acclimatization all affect heat stress.