Heat Index Calculator
Find out how hot it actually feels by combining air temperature and relative humidity. Use this calculator on summer days to assess heat stress and outdoor safety.
About this calculator
The heat index — sometimes called the 'apparent temperature' or 'feels-like' temperature — quantifies how hot the air feels to the human body when humidity is factored in. High humidity slows the evaporation of sweat, which is the body's primary cooling mechanism, making the same air temperature feel hotter. The calculator uses the Rothfusz regression equation developed by the U.S. National Weather Service, valid for temperatures above 80 °F and relative humidity above 40%: 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, where T is air temperature in °F and RH is relative humidity in %. The result is the heat index in °F. Values above 103 °F indicate dangerous heat stress.
How to use
Suppose it is 95 °F outside with 60% relative humidity. Plug T = 95 and RH = 60 into the Rothfusz equation: HI = −42.379 + (2.04901523 × 95) + (10.14333127 × 60) − (0.22475541 × 95 × 60) − (0.00683783 × 95²) − (0.05481717 × 60²) + (0.00122874 × 95² × 60) + (0.00085282 × 95 × 60²) − (0.00000199 × 95² × 60²). Working through the terms gives a heat index of approximately 114 °F — classified as 'extreme danger' by the NWS. Enter your values and the calculator handles all arithmetic instantly.
Frequently asked questions
What is the difference between heat index and air temperature?
Air temperature is a direct physical measurement of how hot the air is, typically taken in the shade with calibrated instruments. Heat index incorporates relative humidity to estimate how hot the human body perceives that air, because our cooling depends on sweat evaporation. At 90 °F with 90% humidity, the heat index can exceed 120 °F, even though the thermometer still reads 90 °F. The gap between air temperature and heat index grows larger as humidity increases, which is why a hot day in Florida feels far more oppressive than the same temperature in a desert.
When is the heat index dangerous and what are the NWS warning thresholds?
The U.S. National Weather Service classifies heat index values into four danger categories. A heat index of 80–90 °F is 'caution' — fatigue is possible with prolonged exposure. From 91–103 °F is 'extreme caution,' where heat cramps and heat exhaustion are possible. From 103–124 °F is 'danger,' where heat cramps, heat exhaustion are likely and heat stroke is possible. Above 125 °F is 'extreme danger,' where heat stroke is imminent. Outdoor workers, elderly individuals, and people without air conditioning are most at risk and should limit exertion during the hottest parts of the day.
Why does humidity make hot weather feel so much worse?
The human body cools itself primarily through sweating: as sweat evaporates from the skin, it carries heat away. Evaporation rate depends on how much water vapor is already in the air — that is, relative humidity. When humidity is high, the air is already saturated with moisture and cannot absorb sweat efficiently, so evaporative cooling slows dramatically. This causes body temperature to rise faster and makes the same air temperature feel significantly hotter. A dry 100 °F day can feel tolerable while a humid 90 °F day can be life-threatening for that very reason.