Heat Index Calculator
Free heat index calculator using the NWS formula. Calculate the apparent temperature based on air temperature and humidity instantly.
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Calculate the Real Feel Temperature
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.
What Is Heat Index?
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...Why Calculate Heat Index?
Heat Safety
Know when outdoor conditions become dangerous for work or exercise.
Activity Planning
Schedule outdoor activities during safer heat index periods.
Health Protection
Protect vulnerable individuals from heat-related illness.
How to Use This Calculator
Common Applications
Outdoor Work
OSHA recommends monitoring heat index for worker safety protocols.
Sports & Athletics
Coaches use heat index to determine safe practice conditions.
Event Planning
Plan outdoor events with attendee comfort in mind.
Elderly Care
Monitor conditions for heat-sensitive individuals.
Frequently Asked Questions
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.