Introduction
The Heat Index Calculator helps you find out what the temperature actually feels like outside. When the air is hot and humid, your body has a harder time cooling itself through sweat. This makes it feel much hotter than the thermometer shows. The heat index combines the air temperature and the relative humidity to give you a single number — the "feels like" temperature. This number is important because high heat index values can be dangerous and lead to heat exhaustion or heat stroke. Simply enter the current temperature and humidity level, and the calculator will show you the heat index right away. Use this tool to stay safe and plan ahead on hot, sticky days.
How to Use Our Heat Index Calculator
Enter the air temperature and relative humidity below to find out what the temperature actually feels like outside. The calculator will give you the heat index, also known as the "feels like" temperature.
Air Temperature: Type in the current air temperature. This is the temperature you see on a thermometer. You can enter it in degrees Fahrenheit or Celsius, depending on what you prefer.
Relative Humidity: Enter the relative humidity as a percentage. This tells the calculator how much moisture is in the air. You can find this number from your local weather report or a home weather station. Humidity must be between 0% and 100%.
If you know the dew point temperature instead of the relative humidity, switch to the "Temp + Dew Point" tab. The calculator will convert the dew point to relative humidity automatically. You can also use our Dew Point Calculator to explore the relationship between temperature, humidity, and dew point in more detail.
What Is the Heat Index?
The heat index is what the temperature actually feels like to your body when you combine the air temperature with the humidity. You might hear people call it the "apparent temperature" or "feels-like temperature." On a hot, humid day, your body has a harder time cooling itself through sweat because the moisture in the air slows down evaporation. This makes it feel much hotter than the thermometer says.
The heat index is the warm-weather counterpart to wind chill, which measures how cold it feels when wind strips heat from your body during cold weather. Together, these two metrics give you a complete picture of how weather conditions actually affect your body year-round.
Why Does the Heat Index Matter?
The heat index is important because it tells you how dangerous the heat really is. When the heat index is high, your body can overheat quickly. This can lead to serious health problems like heat exhaustion or heat stroke, which can be life-threatening. Children, older adults, and people who work or exercise outdoors are especially at risk.
Knowing the heat index is especially valuable for athletes and anyone who exercises outdoors. If you're a runner training in the summer, checking the heat index before heading out is just as important as planning your running pace or marathon pace. High heat index values mean you should slow down, hydrate more, and consider shortening your workout. Swimmers may want to move training outdoors on high heat index days — our Swimming Pace Calculator can help you track pool performance as an alternative to heat-exposed exercise.
Heat Index Danger Levels
- 80–90°F (27–32°C): Caution — fatigue is possible with long exposure.
- 90–103°F (32–39°C): Extreme Caution — heat cramps and heat exhaustion are possible.
- 103–124°F (39–51°C): Danger — heat cramps and heat exhaustion are likely. Heat stroke is possible.
- 125°F (52°C) and above: Extreme Danger — heat stroke is highly likely.
How Is the Heat Index Calculated?
The heat index is calculated using a formula developed by the National Weather Service (NWS). It takes the current air temperature and relative humidity and plugs them into a regression equation. The formula works best when the temperature is at least 80°F (27°C) and the relative humidity is at least 40%. Below those values, the heat index is roughly equal to the actual air temperature.
The NWS algorithm uses the Rothfusz regression equation with additional adjustments for extreme low-humidity and high-humidity conditions. The calculator also computes derived meteorological values such as the dew point, wet bulb temperature, vapor pressure, and absolute humidity — all of which help paint a fuller picture of atmospheric moisture conditions.
Tips for Staying Safe in High Heat
When the heat index is high, drink plenty of water, stay in the shade or air conditioning when possible, wear light and loose clothing, and avoid heavy outdoor activity during the hottest parts of the day. Always check the heat index before spending time outside — the actual temperature alone does not tell the full story. Use our Water Intake Calculator to make sure you're drinking enough fluids, especially when heat and humidity are elevated.
If you're planning outdoor work or construction projects during hot weather, the heat index should guide your work-rest schedule. Whether you're pouring concrete, laying asphalt, or installing decking, high heat index conditions reduce worker productivity and increase the risk of heat-related illness. Monitor the calories you're burning during strenuous outdoor work and take breaks accordingly.
Proper nutrition and monitoring your health metrics also play a role in heat safety. Tracking your heart rate zones during outdoor exercise helps you recognize when your body is overheating before symptoms become serious. On extreme heat days, consider moving workouts indoors and using tools like the BMR Calculator or TDEE Calculator to adjust your energy needs based on reduced activity levels.