Heat Index Calculator
Calculate the "feels like" temperature based on heat and humidity.
"Feels Like" Temperature
Enter conditions to calculate
Heat Index Chart
Source: U.S. National Weather Service
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Calculate the 'Feels Like' Temperature
Our Heat Index Calculator determines how hot it really feels by combining air temperature and relative humidity, helping you understand the risk of heat-related illness.
What is a Heat Index Calculator?
A Heat Index Calculator is a tool that computes the apparent temperature, or what the temperature feels like to the human body when relative humidity is combined with the air temperature. High humidity reduces the body's ability to cool itself through sweating, making it feel hotter than the actual air temperature. This calculator is essential for assessing the risk of heat-related illnesses like heat exhaustion or heatstroke during hot weather.
How It Works: The Heat Index Formula
The calculator uses a complex multiple regression equation developed by the National Weather Service, often referred to as the Steadman formula. A simplified version is:
HI = c₁ + c₂T + c₃R + c₄TR + c₅T² + c₆R² + ...Where T is temperature and R is relative humidity, with multiple coefficients (c₁...cₙ) to adjust the value.
- Enter Air Temperature: Input the current temperature in Fahrenheit or Celsius.
- Enter Relative Humidity: Input the current relative humidity percentage.
- Calculate: The tool instantly provides the "feels like" temperature, or Heat Index.
Interpreting the Results: Understanding Heat Risk
The resulting **Heat Index** value is categorized into risk levels. A value of 90°F to 103°F indicates a need for extreme caution, as prolonged exposure can lead to heat exhaustion or cramps. A value above 103°F is considered dangerous, significantly increasing the risk of heatstroke.
Common Heat Index Myths
- Myth 1: Heat index is the same in the sun and shade. The heat index is calculated for shady, light-wind conditions. Direct sunlight can increase the 'feels like' temperature by up to 15°F (8°C).
- Myth 2: If I'm not sweating, I'm not overheating. In very high humidity, sweat does not evaporate effectively, which is the body's primary cooling mechanism. You can be overheating dangerously even with minimal visible sweat.
- Myth 3: A high heat index is only a problem for the elderly. While older adults are more vulnerable, heat-related illnesses can affect anyone, including young, healthy athletes, outdoor workers, and children.
Frequently Asked Questions
How do you calculate the heat index?
The heat index is calculated using a complex formula that combines air temperature and relative humidity. Our Heat Index Calculator uses the Steadman formula, a widely accepted algorithm, to provide an accurate 'feels like' temperature.
What is a dangerous heat index?
A heat index between 90°F and 103°F (32°C to 39°C) requires extreme caution. A heat index above 103°F (39°C) is considered dangerous, and a value above 124°F (51°C) is extremely dangerous, with a high risk of heatstroke.
Does wind affect the heat index?
No, the standard heat index calculation does not account for wind. Wind can help sweat evaporate faster, providing a cooling effect that the heat index doesn't measure. For cold weather, the 'wind chill' is the equivalent concept.
Is heat index the same as 'feels like' temperature?
Yes, 'heat index' and 'feels like' temperature are often used interchangeably to describe how hot it really feels when relative humidity is combined with the air temperature.
Tips for Staying Safe in High Heat
- Stay Hydrated: Drink plenty of water throughout the day, even before you feel thirsty.
- Seek Air Conditioning: Spend as much time as possible in air-conditioned environments.
- Avoid Strenuous Activity: Limit outdoor exercise and heavy work to cooler parts of the day, like early morning or late evening.
- Wear Appropriate Clothing: Choose lightweight, light-colored, and loose-fitting clothing.
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