Calculate heat index (feels-like temperature) from air temperature and relative humidity.
Reviewed by the CalculatorKosh Editorial TeamUpdated June 2026Free · No sign-up
Heat Index Calculator
Calculate heat index (feels-like temperature) from air temperature and relative humidity.
Heat index applies at ≥27°C (80°F)
How It Works
The heat index — also called the "apparent temperature" or "feels like" temperature — combines air temperature and relative humidity to describe how hot it actually feels to the human body. High humidity prevents sweat from evaporating efficiently, which is your body's primary cooling mechanism. When sweat can't evaporate, your body temperature rises, making hot days feel significantly hotter than the thermometer reads.
Rothfusz Regression Formula (US-NWS)
This calculator uses the Rothfusz regression equation, the formula used by the US National Weather Service (NWS). The physics is universal — it works just as well for Indian conditions like a Delhi summer (45–48°C, low humidity) or a Chennai monsoon afternoon (38–42°C, high humidity). It's valid for air temperatures at or above 27°C (80°F) and relative humidities above 40%. For lower temperatures or humidities, the perceived temperature is close enough to actual temperature that the heat index formula isn't meaningful.
Heat-Related Illness Categories
Caution (27–32°C / 80–90°F): Fatigue possible — typical Mumbai / Bengaluru daytime. Extreme Caution (33–39°C / 91–103°F): Heat cramps and exhaustion possible — typical Delhi / Hyderabad summer afternoon. Danger (40–51°C / 103–124°F): Heatstroke possible — typical Bhubaneswar 40°C with 80% humidity, or Ahmedabad / Nagpur 45°C with moderate humidity. Extreme Danger (52°C+ / 125°F+): Heatstroke highly likely — a life-threatening situation requiring immediate action.
Frequently Asked Questions
Temperature is the actual measure of air heat. Heat index is how hot it feels when humidity is factored in. On a 35°C day with 60% humidity (typical Mumbai monsoon), the heat index is about 45°C — a 10°C difference. On a dry Jaisalmer day (45°C, 20% humidity), the heat index is close to actual temperature. Humidity makes a dramatic difference in comfort and heat stress — which is why Indian coastal cities feel worse than inland desert cities even at lower temperatures.
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