Heat Index Calculator

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An image illustrating the step-by-step process of using a Heat Index Calculator. The image shows a user entering the temperature (in Fahrenheit or Celsius) and relative humidity (in percentage) into the calculator's input fields. The next step displays the calculator automatically calculating the heat index, which is a measure of how hot it feels due to the combined effects of temperature and humidity. This image visually guides the user on how to use the Heat Index Calculator, a helpful tool for assessing the perceived heat and potential risks associated with high temperature and humidity levels.

By factoring in the combination of temperature and humidity, the heat index calculator provides valuable insights and helps you make informed decisions when you need to assess the true level of heat stress.

What is the Heat Index?

The heat index, also called apparent temperature, combines air temperature and humidity into a single "feels like" metric representing perceived warmth. The result gauges the total heat stress and risks.

For example, 90°F with extreme humidity has a much higher heat index and greater danger than 90°F with average moisture levels. Understanding true excess heat conditions allows you to take appropriate safety measures as you work and play outdoors.

Why Calculate Heat Index?

Here are key reasons for tracking this vital health indicator:



Assess risk

Heat index directly indicates level of heat exposure dangers from temperatures and moisture

Trigger alerts

Compare index values against cautionary thresholds to prompt preventative safety breaks and hydration

Quantify stress

Allows communicating humidity's exponential impact on heat stress experienced

Improve models

Feed index data into forecasting systems to better predict heatwaves and guide emergency planning

Let's examine how heat index calculators derive these insights crucial for health and productivity.

Using a Heat Index Calculator

Luckily heat index is easy to determine yourself:

  1. Enter Temperature
    • Typically in °F or °C from a thermometer
  2. Input Humidity %
    • Gathered from a hygrometer reading
  3. Get Heat Index
    • The calculator combines metrics into experienced temp

That's it! Now use the heat index value to guide water intake, work schedules, and safety protocols.

Applying the Information

Once you know the current heat index ranges, take appropriate actions:

  • 80-90°F: Fatigue possible with prolonged exposure and activity
  • 90-105°F: Sunstroke, heat cramps, and exhaustion become likely
  • 105-130°F: Heatstroke highly probable with continued exertion; schedule regular cool-down breaks
  • >130°F: Very high risk of heat stroke and heat exhaustion after only limited time outside

Heat Index Formula

While heat index calculators automate the process, the underlying formula combines temperature and humidity into an apparent temperature.

The heat index equation leverages a regression fit against empirical physiological studies on heat exposure impacts. The key inputs are:

  • T = Air Temperature (Fahrenheit)
  • RH = Relative Humidity (percentage)

The formula produces an adjusted temperature reflecting perceived heat stress.

The Math

Without getting too technical, the general form is:

Heat Index = -42.379 + 2.04901523T + 10.14333127RH - .22475541TRH - .00683783T2 - .05481717RH2 + .00122874T2RH + .00085282TRH2 - .00000199T2RH2

Don't worry about memorizing that! Just leverage the convenient online heat index calculator sites.

Heat Index Warning Levels

Heat Index Range

Risk Level

Health Hazards & Precautions

80°F - 90°F


Possible fatigue with prolonged exposure and activity. Basic heat safety precautions are recommended.

90°F - 105°F

Extreme Caution

Sunstroke, muscle cramps, and heat exhaustion become likely. Additional rest breaks and hydration are needed.

105°F - 130°F


High likelihood of heat illness with extended exertion. Rigorous safety protocols should be activated and enforced.

Above 130°F

Extreme Danger

Heatstroke is highly probable with continued exertion. Physical activity should be halted; seek temperature-controlled shelter.

Frequently Asked Questions (FAQs)

Q1: What is the Heat Index?

The Heat Index is a measure that combines air temperature and relative humidity to assess how hot it feels to the human body. It takes into account the impact of humidity on the body's ability to cool itself.

Q2: How is the Heat Index calculated?

The Heat Index is calculated using specific formulas that consider the air temperature and relative humidity. These formulas help determine the apparent temperature or perceived heat that takes into account the effects of high humidity on the body.

Q3: Why is the Heat Index important?

The Heat Index is important because it provides valuable information about the potential risks of heat-related illnesses, such as heat exhaustion or heat stroke. 

Read More: What is the heat index? - National Weather Service

                    How To Use Heat Index Calculator In Celsius