Science
Heat Index Calculator
Calculate how hot it feels outside by combining air temperature and relative humidity.
How to Use
- Select your preferred temperature scale (Fahrenheit or Celsius).
- Enter the current Air Temperature.
- Enter the current Relative Humidity percentage.
- The calculator determines what the temperature actually feels like to the human body and assesses the danger level.
Formula
Steadman's Heat Index Formula (NWS):
HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T² - 0.05481717R² + 0.00122874T²R + 0.00085282TR² - 0.00000199T²R²
Where T is temperature in °F and R is relative humidity %. Multiple adjustments are applied for extremes.
HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T² - 0.05481717R² + 0.00122874T²R + 0.00085282TR² - 0.00000199T²R²
Where T is temperature in °F and R is relative humidity %. Multiple adjustments are applied for extremes.
The Heat Index Calculator replicates the algorithms used by the National Weather Service to determine the "feels like" temperature. By combining air temperature and relative humidity, it provides critical warnings about heat exhaustion and stroke risks.