Heat Index Calculator Online

Last updated on by Editorial Staff

Heat Index Calculator

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Quick Guide to Use the Heat Index Calculator

  • Enter the temperature in the provided field.
  • Select the temperature unit (Celsius or Fahrenheit).
  • Enter the relative humidity in % in the respective field.
  • Click the “Calculate” button to obtain the Heat Index in the selected Unit.
  • The result will display the Heat Index value along with an alert indicating potential risks.

Note: Ranges for the alert are as follows

Temperature in °CTemperature in °FAlert
27-32°C80-90°FCaution: Extended exposure and activity may lead to fatigue. Persistent activity may increase the risk of experiencing heat cramps.
32-41°C90-105°FExtreme Caution: The potential for heat cramps and heat exhaustion exists. Continued activity may lead to heat stroke.
41-54°C105-130°FDanger: Likelihood of heat cramps and heat exhaustion; continued activity increases the probability of heat stroke.
Over 54°C Over 130°FExtreme Danger: Imminent onset of heat stroke.


For Fahrenheit

heat index calculator


  • HI is the Heat Index in degrees Fahrenheit
  • T is the temperature in degrees Fahrenheit
  • H is the relative humidity as a percentage

For Celsius



  • HI is the heat index in degrees Celsius
  • T is the temperature in degrees Celsius
  • R is the relative humidity

c1, c2, c3, …c9 are constants

C1 = -8.78469475556;

 C2 = 1.61139411;

 C3 = 2.33854883889;

 C4 = -0.14611605;

C5 = -0.012308094;

 C6 = -0.0164248277778;

C7 = 2.211732e-3;

C8 = 7.2546e-4;

C9 = -3.582e-6;

What is the Heat Index?

The Heat Index, often referred to as the “apparent temperature,” represents the perceived warmth experienced by individuals when considering both air temperature and humidity levels.

By incorporating humidity into the equation, the Heat Index provides insight into the actual sensation of heat on the body, aiding in the evaluation of potential heat-related health hazards during extended periods of exposure.

Who Can Use This Calculator?

Anyone, including individuals, outdoor workers, athletes, and event organizers, can use this calculator to assess the heat-related risks based on temperature and humidity.

Industries That Can Use This Calculator

  • Construction and Outdoor Work
  • Athletics and Sports Events
  • Agriculture
  • Emergency Response Services
  • Event Planning

Benefits of Using This Calculator

  • Risk Assessment: Evaluate the risk of heat-related illnesses.
  • Safety Planning: Plan outdoor activities considering potential heat stress.
  • Health Protection: Safeguard individuals from heat-related health issues.
  • Industry Compliance: Meet safety standards in various industries.


How is the Heat Index calculated?

The heat Index is calculated using temperature and humidity values in a specific formula, providing an apparent temperature.

Why is humidity crucial in heat assessment?

Humidity affects the body’s ability to cool itself; high humidity makes it harder for sweat to evaporate, impacting perceived temperature.


The Heat Index Calculator is a valuable tool for individuals and industries to gauge heat-related risks, ensuring safety and well-being during various activities.

Regular use of this calculator contributes to informed decision-making and promotes a culture of safety in diverse settings.