Heat Index Calculator
Calculate the heat index from temperature and relative humidity.
Runs 100% in your browser — nothing is uploaded.
Result
Formula
Rothfusz heat index regression (°F, % RH); applies above ~80 °F / 40% RH.
About this calculator
The Heat Index Calculator estimates the apparent temperature—how hot it actually feels—by combining air temperature with relative humidity. It is used to assess heat-related discomfort and risk during warm weather. The calculator uses the Rothfusz heat index equation, a widely recognized regression model developed by the U.S.
How to use
- 1
Enter the air temperature in °F in the 'Temperature' field.
- 2
Enter the relative humidity in % in the 'Relative humidity' field (0–100).
- 3
Click Calculate to see the heat index in °F and °C, plus the advisory level.
Frequently asked questions
Why does humidity make it feel hotter?
When humidity is high, the air is already saturated with moisture, so sweat evaporates more slowly. Evaporation of sweat is the body's primary cooling mechanism; when it is impeded, the body retains more heat, making it feel hotter than the actual air temperature.
What is the difference between heat index and actual temperature?
Actual temperature is the physical measure of air warmth. Heat index is an apparent temperature that accounts for humidity, reflecting how hot it feels to the human body. It can be significantly higher than the actual temperature in humid conditions.
Is the heat index the same as the 'feels like' temperature?
In many contexts, 'feels like' temperature includes wind chill in cold weather and heat index in warm weather. However, other factors like sunlight and wind can also affect perceived temperature. This calculator specifically uses the heat index formula for hot, humid conditions.
Please note: Results are estimates for general information only and are not professional or medical/financial advice.