About the Heat Index Calculator
The heat index describes how hot the air feels to your body when humidity is taken into account. High humidity slows the evaporation of sweat, which is how the body cools itself, so 90 °F at 70 percent humidity feels like about 106 °F. Enter the air temperature and relative humidity in Fahrenheit or Celsius, and this calculator uses the US National Weather Service formula to give the heat index, the same value in the other unit and a danger level from Caution to Extreme danger, with a short note about the risks.
Updated October 2026
How to use the Heat Index Calculator
- 1Choose Fahrenheit or Celsius.
- 2Enter the air temperature measured in the shade.
- 3Enter the relative humidity as a percentage.
- 4Read the feels-like temperature and the danger level.
- 5Press Copy result to share the figure.
Reading the danger levels
- 80 to 90 °F (27 to 32 °C): Caution, fatigue is possible with prolonged activity
- 90 to 103 °F (32 to 39 °C): Extreme caution, heat cramps and exhaustion are possible
- 103 to 125 °F (39 to 52 °C): Danger, exhaustion is likely and heat stroke possible
- 125 °F (52 °C) and above: Extreme danger, heat stroke highly likely
Staying safe in the heat
Drink water before you feel thirsty, take breaks in the shade or air conditioning, and wear light, loose clothing. Schedule hard work and exercise for the cooler parts of the day. Check on older neighbours and never leave children or pets in a parked car. If someone has a high body temperature, confusion or stops sweating, treat it as an emergency and call for help.
How the calculator treats low temperatures
The full formula is built for temperatures of about 80 °F (27 °C) and above. For cooler air the calculator uses a simpler approximation, as the weather service does, and the result is close to the actual temperature. A heat index below 80 °F is shown as being below the heat index range rather than as a warning.
Related tools
Humidity also drives the Dew Point Calculator, which shows how muggy the air feels. In cold weather use the Wind Chill Calculator. The Temperature Converter changes between scales, the Water Intake Calculator suggests how much to drink, and the Calories Burned Calculator is useful for planning exercise.
Features
- US National Weather Service heat index formula
- Fahrenheit and Celsius
- Danger level from Caution to Extreme danger
- Plain-language note on health risks
- Both units shown together
- Checks for impossible humidity and temperatures
- Runs in your browser
Frequently asked questions
What is the heat index?
The heat index, also called the apparent temperature, combines air temperature and relative humidity to estimate how hot it feels to the human body in the shade. It is used in weather forecasts and heat warnings, especially in humid climates.
How do I calculate the heat index?
Weather services use a regression equation from the US National Weather Service with adjustments for very dry and very humid conditions. This calculator applies that method to the temperature and humidity you enter, so you do not need to work it out by hand.
Why does humidity make it feel hotter?
Your body cools by evaporating sweat. When the air is already full of moisture, sweat evaporates slowly, so less heat leaves your body and you feel hotter. In dry air the opposite is true, and the heat index can be lower than the actual temperature.
At what heat index is it dangerous?
The US National Weather Service uses Caution from 80 °F (27 °C), Extreme caution from 90 °F (32 °C), Danger from 103 °F (39 °C) and Extreme danger from 125 °F (52 °C). The calculator shows which band you are in. Take care earlier if you are older, very young, ill or working outdoors.
Does the heat index apply in direct sunlight?
The values assume shade and light wind. Standing in full sun can raise the effective heat index by up to about 15 °F (8 °C), and strong hot wind can make conditions worse. Treat the result as a minimum in those cases.
What is the difference between heat index and wind chill?
The heat index applies in warm, humid weather and shows how much hotter it feels. Wind chill applies in cold, windy weather and shows how much colder it feels. Each formula works only in its own range of conditions.