Heat Index Calculator
Calculate heat index and heat-related illness risk.
Weather Conditions
°F (Fahrenheit)
% (0-100)
Heat Index
Heat Index
99.7°F
37.6°C
Feels like
Extreme Caution: Heat cramps, heat exhaustion possible
Conditions:
Temperature: 90°F
Humidity: 60%
About This Calculator
The Heat Index Calculator calculates the "feels like" temperature when humidity is combined with air temperature. Heat index is the perceived temperature that the human body feels when relative humidity is combined with air temperature, making it feel hotter than the actual air temperature.
Heat index is most meaningful when the temperature is above 80°F (27°C) and relative humidity is above 40%. High heat index values indicate increased risk of heat-related illnesses such as heat exhaustion and heat stroke.
Formula & Calculation Method
Heat Index Formula:
Rothfusz Equation:
HI = -42.379 + 2.04901523×T + 10.14333127×R
- 0.22475541×T×R - 0.00683783×T²
- 0.05481717×R² + 0.00122874×T²×R
+ 0.00085282×T×R² - 0.00000199×T²×R²
Where:
HI = Heat Index (°F)
T = Air temperature (°F)
R = Relative humidity (%)
Conditions:
Valid for: T ≥ 80°F and R ≥ 40%
The heat index formula (Rothfusz equation) calculates how hot it feels when humidity is factored in with air temperature. High humidity prevents sweat from evaporating effectively, reducing the body's ability to cool itself, making it feel hotter.
How to Use This Calculator
- Enter Temperature: Input the air temperature in Fahrenheit (must be 80°F or higher).
- Enter Humidity: Input the relative humidity percentage (must be 40% or higher for meaningful results).
- View Results: See the heat index temperature and heat-related illness risk warnings.
- Take Precautions: Use the heat index value to protect against heat-related illnesses.
Frequently Asked Questions
What is heat index?
Heat index is the "feels like" temperature when relative humidity is combined with air temperature. High humidity prevents sweat from evaporating, reducing the body's ability to cool itself, making it feel hotter than the actual air temperature.
When is heat index applicable?
Heat index is most meaningful when the air temperature is 80°F (27°C) or higher and relative humidity is 40% or higher. At lower temperatures or humidity, the heat index is less significant.
What are the health risks?
High heat index values increase the risk of heat-related illnesses: 80-90°F: caution (fatigue possible), 91-105°F: extreme caution (heat cramps/exhaustion possible), 105-130°F: danger (heat stroke possible), 130°F+: extreme danger (heat stroke highly likely).
How can I protect against high heat index?
Protect yourself by: staying hydrated, avoiding strenuous activity during peak heat, taking breaks in shade or air conditioning, wearing light-colored, loose-fitting clothing, and being aware of signs of heat exhaustion and heat stroke.
NumCalculators Editorial Team
Multi-disciplinary Expert Team
The NumCalculators Editorial Team consists of subject matter experts across various fields who review and verify the accuracy of our calculators. Each calculator undergoes rigorous testing and validation before publication.