What the heat index measures
Heat index combines air temperature and relative humidity to estimate how hot the environment feels to the human body. Humidity matters because sweat cools you by evaporation. When the air is already moist, evaporation slows, so the same thermometer reading can create much higher physiological stress.
Visual rule of thumb
How this calculator works
The calculator follows the NOAA/National Weather Service approach used for public heat-index products. It starts with a simple approximation. If the apparent temperature reaches the warm-weather threshold, it uses the Rothfusz regression, then applies the low-humidity or high-humidity correction when those edge conditions apply.
Inputs
- Air temperature in Fahrenheit or Celsius
- Relative humidity from 1% to 100%
- Automatic conversion to the Fahrenheit-based NOAA formula
Outputs
- Heat index in your selected unit
- NOAA-style caution, danger, and extreme danger bands
- Dew point estimate and calculation notes
Reading the risk bands
Example: why humidity changes everything
92°F at 30% RH
Feels near the low 90s. Still hot, but sweat evaporates more effectively.
92°F at 65% RH
Often feels around the low 100s, moving into a more serious caution band.
92°F at 85% RH
The apparent temperature can approach dangerous levels during exertion.
Important limits
- The standard heat index assumes shade and light wind.
- Direct sun can raise the perceived heat load substantially.
- Personal risk varies with age, medications, hydration, health status, clothing, and acclimatization.
- For work crews and athletes, combine heat index with local rules, rest schedules, and medical judgment.
Calculate your local conditions
Enter temperature and relative humidity to get the apparent temperature, risk band, dew point, and formula notes in one place.
Open the Heat Index Calculator →