ConverterHub

Heat Index Calculator

Enter the temperature and humidity to find the heat index and how hot it actually feels.

How it is calculated

The heat index is the temperature it feels like once humidity slows the evaporation of sweat. The tool converts your air temperature from °C to °F (F = C × 9/5 + 32), applies the Rothfusz regression equation used for heat index, a polynomial in temperature and relative humidity (RH, 0 to 100%), and converts the answer back to °C. The category is based on the °C result: below 32 Caution, 32 to under 41 Extreme Caution, 41 to under 54 Danger, 54 and above Extreme Danger. The formula is fitted for hot, humid conditions; at mild temperatures it is not meaningful. Wind, direct sun and individual health are not included.

Worked example

The air is 32 °C with 70% relative humidity. In Fahrenheit that is 32 × 9/5 + 32 = 89.6 °F. Putting 89.6 °F and RH 70 into the regression gives about 104.7 °F, which is 40.4 °C. That sits in the 32 to 41 band, so the category is Extreme Caution: heat cramps and exhaustion are possible with prolonged exposure.

Try this example →

Common mistakes

  • Entering the temperature in °F; the field expects °C and the result would be absurdly high.
  • Typing humidity as a fraction (0.7) instead of a percentage (70).
  • Using it for cool or dry weather; the regression only suits warm, humid conditions.
  • Treating the heat index as the real air temperature; it is a perceived value.
  • Forgetting that sun exposure can raise the felt heat considerably above this figure.

About this calculator

This calculator finds the NWS heat index — the apparent temperature the National Weather Service uses, calculated with the Rothfusz regression from air temperature and relative humidity — since high humidity slows sweat evaporation and makes the air feel hotter than the thermometer says.

Meteorologists and weather services use it to issue heat advisories, and coaches, outdoor workers and event organizers use it to judge real heat risk and plan hydration breaks or rescheduling. Enter the temperature and humidity to see the heat index and its risk category.

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