Heat Index Calculator.

Calculate the apparent "feels-like" temperature accounting for heat and humidity. Uses the NWS Rothfusz regression equation.

Temp Unit

Air Temperature

Relative Humidity (%)

Heat Index Danger Levels

Heat Index	Level	Risk
< 27°C (80°F)	No Risk	No significant risk from heat index.
27–32°C (80–90°F)	Caution	Fatigue possible with prolonged exposure and activity.
32–41°C (90–105°F)	Extreme Caution	Heat cramps and heat exhaustion possible.
41–54°C (105–130°F)	Danger	Heat cramps and exhaustion likely. Heat stroke possible.
> 54°C (130°F)	Extreme Danger	Heat stroke highly likely with continued exposure.

How to calculate the heat index

Enter the air temperature.
Enter the relative humidity.
Read the heat index — what the air actually feels like once humidity stops your sweat evaporating.
Check the danger band; above roughly 40 °C apparent temperature, heat exhaustion becomes a real risk.

About Heat Index

The heat index (also called the "apparent temperature" or "humiture") is the temperature that the human body perceives when relative humidity is factored in alongside actual air temperature. High humidity reduces the rate of sweat evaporation, which is the body's primary cooling mechanism, making it feel hotter than it actually is.

This calculator uses the NWS Rothfusz regression for conditions where the heat index exceeds 80°F (27°C), and a simpler formula below that threshold. The equation was originally developed by George Winterling in 1978 and has since been refined by the National Weather Service.

The heat index is most meaningful when the air temperature is at least 27°C and relative humidity is above 40%. Below these thresholds, the result is shown but may differ from other sources.

Frequently Asked Questions

What is the heat index?

The heat index is how hot it feels once humidity is taken into account, because humid air slows the evaporation of sweat. At 35 °C (95 °F) and 60% relative humidity it feels like about 45 °C (113 °F), ten degrees above the thermometer.

How is the heat index calculated?

With the US National Weather Service's Rothfusz regression, a nine-term polynomial in temperature (°F) and relative humidity, plus the NWS adjustments for very dry and very humid air. Below about 80 °F (27 °C) a simpler Steadman formula is used, because the regression is not valid there.

What heat index is dangerous?

The NWS bands are: 80–90 °F (27–32 °C) caution, 90–105 °F (32–41 °C) extreme caution, 105–130 °F (41–54 °C) danger, and above 130 °F (54 °C) extreme danger, where heat stroke is likely with continued exposure.

Does the heat index account for sun and wind?

No. It assumes shade and a light breeze. Direct sunshine can add up to about 8 °C (15 °F) to the heat index, and strong wind changes it too. Treat it as a floor for how hot conditions feel, not a ceiling.

Sources · Reviewed 24 September 2026

US National Weather Service: the heat index equation (Rothfusz regression)