Free heat index calculator using the NWS formula. Calculate the apparent temperature based on air temperature and humidity instantly.
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The heat index, also known as apparent temperature, tells you how hot it actually feels when humidity is factored in with the air temperature. High humidity prevents sweat from evaporating, making it harder for your body to cool down. Our calculator uses the official NWS formula.
Heat index combines air temperature and relative humidity to determine how hot it feels to the human body. When humidity is high, perspiration doesn't evaporate as efficiently, reducing your body's ability to cool itself. At 90°F with 70% humidity, it can feel like 106°F.
NWS Heat Index Formula
HI = -42.379 + 2.049T + 10.143RH - 0.225T×RH...Know when outdoor conditions become dangerous for work or exercise.
Schedule outdoor activities during safer heat index periods.
Protect vulnerable individuals from heat-related illness.
OSHA recommends monitoring heat index for worker safety protocols.
Coaches use heat index to determine safe practice conditions.
Plan outdoor events with attendee comfort in mind.
Monitor conditions for heat-sensitive individuals.
A heat index of 103°F (39°C) or higher is considered dangerous. Heat exhaustion and heat stroke become likely with prolonged exposure. Above 125°F (52°C) is extremely dangerous.