Enter the air temperature and relative humidity (or dew point) to calculate the Heat Index — what the temperature actually feels like to your body. The calculator returns the apparent temperature, a heat stress category, and a safety advisory so you know when conditions become dangerous.
Heat Index (Feels Like)
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Heat Index in Celsius
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Heat Stress Category
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Risk Level
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The Heat Index — often called the 'feels-like' or 'apparent temperature' — combines air temperature and relative humidity to express how hot it actually feels to the human body. High humidity reduces the body's ability to cool itself through sweat evaporation, making hot days feel even hotter.
This calculator uses the Rothfusz regression equation adopted by NOAA/NWS, which applies when the temperature is at or above 80°F (27°C) and relative humidity is at or above 40%. For lower temperature-humidity combinations, a simpler Steadman formula is used. The equation accounts for eight polynomial terms involving temperature and humidity.
Yes. This calculator supports both input methods. When you select 'Dew Point', it first converts the dew point to relative humidity using the Magnus formula, then applies the standard Heat Index calculation. Dew point is often considered a more absolute measure of atmospheric moisture than relative humidity.
A Heat Index of 91–103°F is considered 'Very Warm' to 'Caution' territory. At 103–124°F, heat exhaustion becomes possible with prolonged exposure. Above 124°F, heat stroke is likely and conditions are considered extremely dangerous. Always seek shade, hydrate frequently, and avoid strenuous activity when the Heat Index is high.
When humidity is high, sweat doesn't evaporate efficiently from your skin. Since evaporation is your body's primary cooling mechanism, reduced evaporation causes body heat to build up faster, making it feel hotter than the thermometer reads.
No. The Heat Index (used mainly in the United States by NOAA/NWS) and the Humidex (used in Canada) are both apparent temperature measures, but they use different formulas and typically produce slightly different values. The Humidex tends to report higher values than the Heat Index for the same conditions.
The standard Heat Index is calculated for shaded conditions. Direct sunlight can add an additional 10–15°F (6–8°C) to the apparent temperature, so actual outdoor feels-like temperatures in direct sun may be significantly higher than the calculated Heat Index.
The Rothfusz Heat Index formula is most accurate when the air temperature is 80°F (27°C) or higher and relative humidity is 40% or higher. Results outside this range may be less reliable. For very low temperatures or very low humidity, the Heat Index is less meaningful because heat stress is not a concern.