How it works
Calculate the ‘feels like’ temperature by combining air temperature and relative humidity.
Input query strings and outputs
Use an input name in this page’s URL as ?name=value, and join additional inputs with &. Portable shared links may instead use the compact ?ac= state parameter.
Input query strings
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?temp=Temperature (°F)Number · Optional · Default: 90
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?humidity=Relative Humidity (%)Number · Optional · Default: 60
Outputs
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resultResultText · Primary output
result contains the calculator’s complete rendered result area, including its visible result cards, tables, charts, and messages.
Calculate the “perceived temperature” based on heat and humidity
Description
The Heat Index Calculator is a safety tool used to find the “Apparent Temperature”—what the air actually feels like to the human body. When humidity is high, the body’s ability to cool itself through sweat is reduced, making the temperature feel much hotter and more dangerous than the thermometer indicates. This tool is essential for athletes, outdoor workers, and parents to assess the risk of heat-related illnesses.
Inputs
- Temperature (°F): The current air temperature (must be at least 80°F for the heat index to be applicable).
- Relative Humidity (%): The current percentage of moisture in the air.
Outputs
- Heat Index: The “feels like” temperature in Fahrenheit.
Chart
- N/A: This tool provides a direct numeric meteorology result.
“Good to Know”
- The Formula: This calculator uses the complex regression equation developed by the National Weather Service (NWS) to approximate the heat index.
- Danger Zones:
- 91°F - 103°F: Caution (fatigue possible with prolonged exposure).
- 103°F - 124°F: Extreme Caution (heat cramps and exhaustion likely).
- > 125°F: Extreme Danger (heat stroke highly likely).
- Shade vs. Sun: Heat index values are calculated for shady, light wind conditions. Exposure to full sunshine can increase heat index values by up to 15°F.
Examples
Example 1: High Humidity
- Input: 90°F, 70% Humidity
- Output: 105.8 °F (Extreme Caution)
Example 2: Dry Heat
- Input: 100°F, 20% Humidity
- Output: 99.1 °F (Actually feels slightly cooler due to very low humidity!)
Example 3: Moderately Humid
- Input: 85°F, 50% Humidity
- Output: 86.8 °F