Heat Index Calculator
Calculate the feels-like temperature based on air temperature and humidity.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
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This calculator determines the "feels like" temperature by combining air temperature with relative humidity. High humidity reduces your body's ability to cool itself through sweat evaporation, making the air feel hotter than the actual temperature. Follow these steps:
- Enter the air temperature. Use the current temperature in degrees Fahrenheit. The heat index formula is most accurate for temperatures at or above 80°F. Below 80°F, the heat index is generally close to the actual air temperature.
- Enter the relative humidity. This is a percentage from 0 to 100. You can find current humidity on any weather app or website. Typical summer humidity ranges from 40% to 80% depending on your location.
- Read your results. The primary result shows the heat index in degrees Fahrenheit. Below it, you will see the danger category (Caution, Extreme Caution, Danger, or Extreme Danger), the associated health risks, and the Celsius equivalent.
Results update instantly as you adjust the inputs. Use the Share button to send a link with your current values, or Copy to grab the result. This calculator uses the National Weather Service regression formula and runs entirely in your browser.
About the Heat Index
The heat index, also called the apparent temperature, is a measure of how hot it actually feels when humidity is factored in with the air temperature. It was developed by Robert G. Steadman in 1979 and adopted by the National Weather Service. The calculation uses a multiple regression equation that accounts for the combined effects of temperature and moisture on the human body.
When the heat index reaches 80°F or higher, the risk of heat-related illness increases. At 105°F or above, heat stroke becomes a serious danger, especially during prolonged exposure or physical activity. Staying hydrated, seeking shade, and limiting outdoor exertion are the primary ways to reduce risk during high heat index conditions.
Frequently Asked Questions
What is the heat index?
The heat index is the "feels like" temperature that combines air temperature and relative humidity into a single value. When humidity is high, sweat evaporates more slowly, reducing your body's ability to cool down. A temperature of 90°F with 70% humidity has a heat index of about 106°F, meaning your body experiences heat stress equivalent to 106°F in dry air.
How is the heat index calculated?
The National Weather Service uses a regression equation developed by Rothfusz (1990) based on Steadman's 1979 research. The formula includes nine terms with coefficients applied to temperature, humidity, and their interactions. For temperatures below 80°F or humidity below 40%, simpler adjustments are applied. This calculator uses the full NWS algorithm.
At what heat index is it dangerous?
The NWS defines four danger levels. Caution (80 to 90°F): fatigue possible with prolonged exposure. Extreme Caution (90 to 103°F): heat cramps and heat exhaustion possible. Danger (103 to 124°F): heat cramps and heat exhaustion likely, heat stroke possible. Extreme Danger (125°F and above): heat stroke highly likely. These thresholds apply to people in the shade; direct sun can add up to 15°F to the heat index.
What is the difference between heat index and temperature?
Temperature measures the actual thermal energy of the air using a thermometer. The heat index accounts for humidity on top of temperature to reflect how hot it feels to the human body. In dry conditions (low humidity), the heat index may be close to or even below the actual temperature because sweat evaporates efficiently. In humid conditions, the heat index can be significantly higher than the air temperature.