Calculators
Heat Index Calculator
Estimate the heat index from air temperature and relative humidity with caution categories locally in your browser.
Enter the air temperature and relative humidity to estimate how hot it feels. Calculations run locally in your browser; your values are not uploaded or stored.
See how hot it actually feels before you head outside.
Enter the air temperature and relative humidity in Fahrenheit or Celsius. The calculator applies the NOAA Rothfusz regression to estimate the heat index, the apparent temperature your body feels, and shows the matching US National Weather Service caution category. This is an atmospheric estimate, not a weather forecast, and your values are calculated locally without being uploaded or stored.
Frequently Asked Questions
Everything you need to know about this tool, how it works, and privacy.
What formula does the heat index calculator use?
It applies the NOAA Rothfusz regression to the air temperature in Fahrenheit and the relative humidity, with low-humidity and high-humidity adjustments from the US National Weather Service. For example, 90 °F at 60 percent relative humidity gives a heat index of about 100 °F.
What is the difference between heat index and air temperature?
Air temperature is measured in the shade, while the heat index is an apparent temperature that accounts for how humidity slows sweat evaporation and makes it harder for your body to cool itself. Higher humidity at the same air temperature produces a higher heat index.
What do the caution categories mean?
Below 80 °F no caution is needed. From 80 to 90 °F is Caution with fatigue possible, 90 to 103 °F is Extreme caution with heat cramps and heat exhaustion possible, 103 to 126 °F is Danger with heat exhaustion likely and heat stroke possible, and 126 °F or higher is Extreme danger with heat stroke likely.
Which inputs are accepted?
Enter an air temperature from -50 to 140 °F (-45 to 60 °C) and relative humidity from 0 to 100 percent. Missing or non-numeric entries show a clear error instead of a result.
Do my values stay private?
Yes. The calculation and copy action run locally in your browser. CodeASystem does not upload or store the values you enter.