Heat Index CalculatorFree online other calculator — instant results, no sign-up
Feels-like temperature from humidity.
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About the Heat Index Calculator
This calculator computes the apparent temperature, or heat index, based on ambient air temperature and relative humidity. It is commonly used by athletes, outdoor workers, and meteorologists to assess the human body's perceived heat exposure.
- Temperature
- Ambient air temperature measured in degrees Celsius (°C), typically between 20 °C and 50 °C for meaningful heat index calculations.
- Relative humidity
- The percentage of water vapor present in the air relative to saturation, ranging from 0% to 100%.
Formula and method
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
- HIHeat index in degrees Fahrenheit (°F)
- TAir temperature converted to degrees Fahrenheit (°F), where T = t × (9/5) + 32
- RRelative humidity as a percentage (e.g., 60 for 60%)
The tool converts the Celsius temperature to Fahrenheit, then evaluates the 9-term Rothfusz polynomial approximation for apparent temperature. The resulting heat index in Fahrenheit is rounded to the nearest integer, and its Celsius equivalent is converted back and rounded to one decimal place.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Temperature32 °C
- Relative humidity60 %
Results
- Heat index37.1 °C (99 °F)
How to use the Heat Index Calculator
- 1Convert the input Celsius temperature to Fahrenheit using T = (t × 9/5) + 32.
- 2Insert T and the relative humidity percentage R into the 9-term polynomial equation.
- 3Evaluate the polynomial to determine the heat index in degrees Fahrenheit.
- 4Convert the Fahrenheit result back to Celsius using (HI - 32) × 5/9 and round to one decimal place.
Reading your result
The result represents how warm the air feels to human skin when evaporative cooling via sweat is impaired by moisture in the air. A heat index higher than the actual ambient temperature indicates that high humidity is reducing the body's rate of cooling.
Limitations
The equation is a polynomial regression model designed for hot and humid conditions (typically temperatures above 26.7 °C / 80 °F and relative humidity above 40%). It assumes shaded conditions with light wind and does not account for direct sunlight or strong airflow, both of which alter perceived heat.
Common questions
Why does high humidity make the air feel hotter?+
The human body cools itself primarily through perspiration. When the air has high relative humidity, sweat evaporates more slowly, reducing cooling efficiency and making the perceived temperature feel higher.
Can the heat index be lower than the actual temperature?+
Yes, at low relative humidity levels, rapid sweat evaporation can cause the calculated heat index to fall slightly below the ambient temperature.
Does this formula apply in cold weather?+
No, the heat index equation is calibrated specifically for warm and hot conditions. For cold environments, the wind chill index is used instead to gauge cold stress.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
Further reading
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Results are estimates for informational purposes and are not financial, medical or legal advice.