Calculate the Heat Index

When is it too hot to exercise outside? What is a safe temperature to not risk heat exhaustion or heat stroke? The first step to find out the answer to these questions is to know the current heat index.

The heat index is what the air temperature feels like to your body. Heat index is calculated using air temperature (in the shade) and relative humidity. These two values determine what the temperature feels like to your body.

To use the heat index calculator, you need to know the air temperature in the shade and the relative humidity. Following our calculator is a chart letting you know the warning levels for exercising in the current heat index. The calculation we use is the same formula used by the National Weather Service of the National Oceanic and Atmospheric Administration.

Relative Humidity:
The heat index is:  

What does the heat index you just calculated mean? It's what the air temperature feels like to your body. So when you hear someone say "it's 110 in Arizona", then of course somebody responds with "but it's dry heat", you'll know what it means... it means it's still HOT. Even at a low 10% relative humidity, 110 degrees feels like 102 degrees.

Use your current heat index to keep yourself safe while exercising outdoors. Our following chart will show you how. But be aware that everybody reacts to exercising in heat differently. What might be unbearably hot to someone else may feel nice and warm to you. So use these heat index charts as a reference only. How you feel during exercise is what's most important.

Heat IndexHeat risk associated with exercise
Less than 90Safe to exercise outdoors
It may be a little warm when your heat index is above 80 so be careful, but enjoy yourself outside. This is a great time to acclimate yourself to the heat, so you're ready for hotter days. Remember, you still need to stay hydrated and watch for any heat related issues.
90-105Heat cramps or heat exhaustion possible
If you're healthy and acclimated to the heat, exercising outside when the heat index is in this level is ok, but be careful. Heat related issues can happen at any time. Stay hydrated and don't over due it.
106-130Heat cramps or heat exhaustion likely. Heat stroke possible
Best to avoid exercising outside as much as possible. If you must, and you already are acclimated to the heat, you still need to be real careful. Swimming is a great activity when the heat index is this high. Drink a lot, stay in the shade, take it easy, and cool down as soon as possible after exercising. Stop immediately and take care of yourself if you feel any heat related issues.
Greater than 131Heat stroke highly likely
Do not exercise outside. A heat related illness can happen real quick at this heat index level and become real serious quickly. Find an indoor activity.

You can also look at the following heat index chart from the NOAA's National Weather Service to figure out your current heat index and determine if it's safe to exercise outside. Remember that these are only general precautions. For some, safe conditions may still be too warm for strenuous outdoors exercise.

Temperature (F)

  Caution    Extreme Caution    Danger    Extreme Danger

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