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Heat Index Calculator

Our free Heat Index Calculator helps you determine how hot it actually feels by combining temperature and humidity values. Enter the air temperature and relative humidity to calculate the apparent temperature and heat risk level.

Features

Accurate Heat Index Calculation

Calculate heat index using the official formula from the National Weather Service for precise apparent temperature results.

Heat Risk Assessment

Get immediate feedback on potential heat-related risks based on the calculated heat index value.

Multiple Temperature Units

Choose between Celsius and Fahrenheit for both input and output temperature values.

How to Use the Heat Index Calculator

Calculate the heat index in just a few simple steps.

  1. 1

    Choose Temperature Unit

    Select your preferred temperature unit (Celsius or Fahrenheit).

  2. 2

    Enter Temperature

    Input the current air temperature in your chosen unit.

  3. 3

    Set Humidity

    Enter the relative humidity using the slider or input field (0-100%).

  4. 4

    Calculate

    Click the 'Calculate Heat Index' button to see the apparent temperature and risk level.

Frequently Asked Questions

The heat index, also known as the 'apparent temperature', is a measure of how hot it actually feels when relative humidity is combined with air temperature. It's important because high humidity reduces the effectiveness of sweating to cool the body, making it feel hotter than the actual temperature.

The heat index is calculated using a complex equation that takes into account temperature and relative humidity. Our calculator uses the official formula developed by the National Weather Service, which includes multiple regression analysis of how the human body perceives different temperature and humidity combinations.

The risk levels indicate potential health concerns: Caution (27-32°C/80-90°F) means fatigue is possible with prolonged exposure; Extreme Caution (32-41°C/90-105°F) means heat cramps and heat exhaustion are possible; Danger (41-54°C/105-130°F) means heat cramps and heat exhaustion are likely with continued activity; Extreme Danger (above 54°C/130°F) means heat stroke is imminent.

Yes, in hot conditions, the 'feels like' temperature reported in weather forecasts is typically the heat index. However, in cold conditions, the 'feels like' temperature usually refers to the wind chill factor instead.