Heat Index Calculator

When humidity climbs, sweat evaporates slowly and a hot day feels even hotter than the thermometer claims. The heat index captures that combined effect of air temperature and relative humidity, giving you the apparent temperature your body has to cope with. It matters because heat is one of the deadliest weather hazards in the United States, with outdoor workers and vulnerable groups bearing more of the burden, and a clear number helps you time outdoor work and stay ahead on hydration before exhaustion or heat stroke becomes a threat.

Behind the result is the National Weather Service heat index equation, a multiple-regression fit to Steadman's apparent-temperature results that Lans P. Rothfusz described in a 1990 NWS Technical Attachment (SR 90-23). It accounts for the way temperature and humidity interact non-linearly to produce the apparent temperature an average person feels. The full equation carries many terms and coefficients; the calculator handles that math from your two inputs and sorts the answer into the standard danger categories.

A few caveats shape how you read the output. The index assumes shade and a light breeze, so standing in direct sun can push the real feel up to 15°F (8°C) higher, and hard physical effort adds still more strain. Tolerance also varies with age, underlying health, and how acclimatized you are to heat, which means a reading that looks merely moderate can still be dangerous for some people, and your own symptoms remain the signal to heed.