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Whether you're studying to become a meteorologist or are just a normal human being that likes to spend their time outside, understanding heat index can be a major benefit. If you're looking to figure out the heat index for vacation destination or have work you need help with, try our free calculator.

On this page, we'll be discussing heat index and how you can use our calculator to measure it. If you find this page helpful, save it to the home screen of your smartphone to get fast access when you need it.


Why use a heat index calculator?

If you want to calculate the heat index of a given day in your area or on vacation, a heat index calculator is the best way to go.

As you will soon see, the equations used to calculate the heat index are aggressively complex.

If you were to calculate the heat index the old-fashioned way, the weather conditions of your area would probably change before you even got done working through the equations.

Also, you don't want to risk making a mistake and venturing out into weather conditions that are more dangerous than you thought because of one flub in your arithmetic.

What is heat index and why is it important?

Heat index is a measure of the heat on any given day that takes into account the actual temperature outside as well as the humidity or amount of moisture in the air.

This is done because when it's very humid out, high temperatures can feel hotter than it really is.

Why That's Important

This isn't just a psychological phenomenon. It's rooted in a biological understanding of our body composition, too.

Typically, to regulate its internal temperature, our body sweats. With the evaporation of sweat from our skin, our bodies cool down.

But when it's really humid outside, it's harder for that sweat to evaporate. This means our bodies can't cool down as much as they should be, which can be dangerous.

These conditions can lead to dehydration, heat stroke, or worse.

That's why calculating the heat index is so important. When weather forecasters release the heat index, they are letting the public know that it's actually hotter out than it seems and this allows the public to take extra precautions.


A given day could have a temperature of 88 degrees, but if the humidity is 70% the heat index jumps up to 100 degrees.

These are wildly different temperatures, and people spending time outside should be able to plan accordingly.

How do you measure heat index?

To measure and calculate the heat index, you need to have the values for two variables:

  • the air temperature
  • the amount of humidity in the air (this is measured by a percent value)
  • Regression Equation of Rothfusz

    To calculate the heat index value, you have to plug these variables into an equation known as the regression equation of Rothfusz:

    HI = -42.379 + 2.04901523 · T + 10.14333127 · RH - .22475541 · T · RH - .00683783 · T · T - .05481717 · RH · RH + .00122874 · T · T · RH + .00085282 · T · RH · RH - .00000199 · T · T · RH · RH

    It's a bit much, we know. In this equation:

    • HI stands for the heat index
    • the Ts represent the temperature
    • and the RHs represent the percentage of moisture in the air.

    Adjustments To The Equation

    However, this equation alone is not enough. If the temperature and humidity fall within certain ranges, you have to use additional equations to find additional values called "adjustments."

    Humidity Below 13%

    If the RH of a given day is less than 13% and the T is between 80 and 112 degrees Fahrenheit, you have to subtract this adjustment from the HI you calculated using the Rothfusz equation:

    ADJUSTMENT = [(13 - RH) / 4] · SQRT{[17 - ABS(T - 95.)] / 17}

    In this equation, SQRT represents a square root function and ABS represents the absolute value.

    Humidity Above 85%

    For different circumstances, there is yet another adjustment value you have to calculate.

    If the RH of a given day is greater than 85% and the temperature is between 80 and 87 degrees Fahrenheit, this adjustment is added to your initial heat index calculation:

    ADJUSTMENT = [(RH - 85) / 10] · [(87 - T) / 5]

    Heat Index Below 80 Degrees

    Lastly, if you used the Rothfusz equation and calculated a heat index below 80 degrees, you either did it wrong or the RH and T were too low for that equation to be useful.

    In the latter instances, you should use a much simpler equation:

    HI = 0.5 · {T + 61.0 + [(T - 68.0) · 1.2] + (RH · 0.094)}

    Heat Index Chart

    If you find the heat index equations too challenging (we don't blame you), you can also reference a heat index chart.

    Heat Index Chart

    When you look at the above chart, you'll find numbers running down the furthermost vertical column on the left hand side.

    These numbers increase as you go down, corresponding to relative humidity. On the uppermost horizontal column, you'll find temperature readings in Fahrenheit.

    The value where both points intersect is the related heat index, based on data from the National Weather Service. The areas in red respresent extreme danger with prolonged outdoor exposure or strenuous activity.

    Are heat index and ''feels like'' the same?

    The short answer to this question is, yes.

    On weather broadcasts or phone applications, the term heat index and "feels like" are used interchangeably. But it's important to remember, again, that the heat index is measuring an actual thing that affects our bodies.

    The phrase "feels like" often makes people think that it's just a psychological phenomenon when it's not.

    Where is the world's highest heat index?

    When and where the highest heat index ever was recorded is up for debate, as there are no official records for some reason.

    Dhahran, Saudi Arabia

    Allegedly, the highest heat index ever was recorded in Dhahran, Saudi Arabia on July 8th, 2003.

    The heat index was a stunning 178 degrees.

    On that day, the air temperature was an already-stifling 108 degrees with humidity percentage of 68.

    Bandar Mahshahr, Iran

    The second highest, but actually officially recorded, heat index was recorded again in the Middle East in a city called Bandar Mahshahr in 2015.

    The heat index reached a high of 165 degrees with an air temperature of 115 and a moisture percentage around 46.

    If you're planning a trip to the Mid-East anytime soon, take this as a caution against making your visit in the summer.

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