Heat Index Calculator

1. Enter the air temperature. Use the current temperature from a weather service, not the "feels like" temperature already shown. The calculator will compute that for you.
2. Enter relative humidity. Humidity is listed on most weather apps. Higher humidity makes heat more dangerous because sweat cannot evaporate efficiently, reducing your body's cooling ability.
3. Read the heat index and risk category. The NWS uses four risk levels: Caution, Extreme Caution, Danger, and Extreme Danger. These guide decisions about outdoor activity, especially for the elderly, children, and people working outdoors.

The National Weather Service uses the Rothfusz regression equation, which was derived from Steadman's human comfort research:

HI = -42.379
   + 2.04901523 × T
   + 10.14333127 × R
   - 0.22475541 × T × R
   - 0.00683783 × T²
   - 0.05481717 × R²
   + 0.00122874 × T² × R
   + 0.00085282 × T × R²
   - 0.00000199 × T² × R²

Where: T = temperature in °F, R = relative humidity (%)

This formula is valid when T is at or above 80°F and RH is at or above 40%. Outside this range, simpler formulas are used.

Example: T = 95°F, RH = 60%:

HI = -42.379 + 2.049×95 + 10.143×60
     - 0.225×95×60 - 0.00684×9025
     - 0.0548×3600 + 0.00123×9025×60
     + 0.000853×95×3600 - 0.000002×9025×3600
   ≈ 113°F

At 95°F and 60% humidity, the heat index is approximately 113°F, well into the Danger zone.