Heat Index Calculator
Apparent temperature combining heat and humidity.
Formula
HI = Rothfusz polynomial in T(°F), RH(%)
How to use
- 1Enter the air temperature.
- 2Enter the relative humidity (%).
- 3Choose Celsius or Fahrenheit.
- 4Read the heat index, category, health risk and recommended action.
About this calculator
A thermometer reading tells only half the story on a humid day. When the air is damp, sweat cannot evaporate efficiently, so the heat feels far worse than the number suggests. The heat index (apparent temperature) combines air temperature and humidity into one feels-like figure, and this tool adds the health risk and what to do about it.
Formula explained
The calculator uses the Rothfusz regression, the same polynomial the US National Weather Service uses. It is an equation in temperature (°F) and relative humidity with correction terms; Celsius input is converted to °F, the heat index is computed, then converted back. Below about 80°F a simpler linear estimate is used, since the full polynomial targets hot, humid weather.
Worked example
32°C at 70% relative humidity produces a heat index well above 32, landing in the Extreme Caution range - humid heat stresses the body like a much hotter, drier day.
Tips & notes
- Check the heat index, not the raw temperature, before strenuous activity.
- The index assumes shade; add caution in direct sun.
- Hydrate before you feel thirsty and schedule hard work for cooler hours.