Heat Index Calculator

Created by Krishna Nelaturu
Last updated: Aug 30, 2022

Our heat index calculator will help you determine whether it will be a pleasant day to plan your outdoor activities. Any weather forecast is incomplete without heat index data, which tells you what the weather actually feels like. The following article explains why knowing the heat index today is essential. Specifically, we shall look at:

  • What is the heat index?
  • How to calculate feels like temperature?
  • Heat index chart.

What is heat index?

We regulate our body temperature by sweating, which evaporates and cools our bodies. However, humidity in the air impedes this evaporation, making it harder for us to cool down. In other words, how hot (or cool) we perceive the day depends on the air temperature and humidity - the amount of water vapor in the air.

Heat index measures how hot the day feels to us, primarily based on the air temperature and humidity. It is also known as the humidex or feels like temperature.

Usually, this makes us feel like the temperature is higher than the mercury reading. For example, if the temperature forecast is 87°F87 \degree \text{F} on a day with a relative humidity of 84%84\%, your body will perceive it to be 105°F105 \degree \text{F} outside.

Our temperature converter can help you with any temperature conversions necessary.

Heat index chart

The National Oceanic Atmospheric Administration (NOAA) has issued a heat index chart that we can use to conveniently find the heat index from the temperature and relative humidity.

NOAA's heat index chart.
Heat index chart from the NOAA. Math the temperature reading with the relative humidity measurement, and you can find out the humidex.

Heat waves are one of the deadliest weather disasters. Be careful when going outdoors if the heat index is higher than 105 °F because you're at risk of heat stroke. Stay hydrated and avoid prolonged outdoor activities. Sadly, due to climate change, heat waves are going to be more frequent and deadly.

How to calculate heat index

We used an equation for heat index that reproduces the heat index chart data in this humidex calculator, but you can refer to the chart for most cases:

  1. Measure the air temperature outdoors, also known as dry bulb temperature.
  2. Measure the relative humidity of the air. You can calculate that from the air temperature and dew point using our relative humidity calculator.
  3. Look up these values on the heat index chart to determine how hot it feels.

How to use this heat index calculator

As the saying goes, humidity makes the heat worse. At least with our heat index calculator, you can find precisely how much worse.

  • Enter the temperature and relative humidity to calculate the heat index.
  • You can use the temperature and dew point if you don't know relative humidity.
Krishna Nelaturu
Relative Humidity
Dew point
Heat index
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