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Heat Index Calculator

The heat index is the "feels like" temperature when humidity is combined with air temperature. It helps you judge heat-related risk.

How it works Report a problem

Heat Index Calculator

Rothfusz regression — perceived "feels like" temperature

Rothfusz multi-variable regression formula (NWS standard)
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Heat Index (Feels Like)
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In °C
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In °F
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Hotter Than Actual
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On a humid day the thermometer understates the stress on your body, because sweat cannot evaporate into air that is already nearly full of vapour. The heat index corrects the temperature for humidity, giving the figure your body actually experiences. This calculator computes it from the two measurements.

How to use this calculator

  • Enter the air temperature.
  • Enter the relative humidity.
  • Press calculate to see the heat index (feels-like temperature).

Where the danger bands start

Heat index values carry official caution bands: prolonged exposure above roughly 32 °C (90 °F) calls for caution, above 39 °C (103 °F) is dangerous, and above 52 °C (125 °F) is life-threatening. Full sun can add several degrees to the calculated figure, which is why shade readings still underestimate a roofer's or runner's real load.

Frequently asked questions

How is the heat index calculated?
From a regression formula fitted to how humans perceive temperature and humidity together, used by weather services. It applies once temperatures are warm enough (about 27 °C / 80 °F) for humidity to matter.
Why does humidity make heat feel worse?
Cooling depends on sweat evaporating. Humid air accepts little extra vapour, so sweat lingers, cooling stalls, and body temperature climbs instead.
Is heat index the same as wind chill?
They are counterparts: heat index adjusts hot temperatures upward for humidity; wind chill adjusts cold temperatures downward for wind. Neither changes the actual temperature.