Heat Index Calculator

Heat index is the apparent temperature your body feels when air temperature and humidity combine. At 85°F with high humidity, your body perceives it much hotter because sweat cannot evaporate efficiently. It's a measure of thermal comfort and health risk.

Your body cools itself through sweat evaporation. High humidity prevents sweat from evaporating, trapping heat against your skin. This is why a dry 95°F feels more tolerable than a humid 85°F—humidity can make a larger difference than temperature alone.

Heat index depends only on temperature and relative humidity at your location, so it varies by place and time. Factors like wind, shade, and solar radiation are not included in the formula—direct sun can make the real-world feel 15°F hotter than the calculated index.

Precautions depend on activity level and health. At Caution level (80–90°F), limit intense exercise and drink water frequently. At Danger level (104°F+), avoid outdoor activity entirely and stay indoors in air conditioning. Children, elderly, and those with health conditions are at higher risk.

Yes. The formula returns the air temperature itself when humidity is below 40%, since evaporative cooling is already efficient. In deserts with low humidity, the heat index may be close to or equal to the actual temperature.

The NOAA (National Oceanic and Atmospheric Administration) and the NWS (National Weather Service) define heat danger thresholds based on medical research and heat-related illness data. These thresholds guide public health warnings and safety recommendations.