Calculate the heat index ("feels like" temperature) from air temperature and relative humidity, using the official NOAA formula.
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The heat index, often described as the "feels like" temperature, measures how hot the human body actually perceives the air to be when both air temperature and relative humidity are taken into account together, since high humidity significantly reduces the body's ability to cool itself through sweat evaporation. This tool uses the official regression formula published by the U.S. National Weather Service (NOAA), derived from a more complex biometeorological model, along with the small additional adjustments NOAA specifies for unusually low humidity at high temperatures and unusually high humidity at more moderate high temperatures. Below 80°F, the heat index is generally considered equal to the actual air temperature, since humidity has little meaningful impact on perceived heat at lower temperatures. This is the same formula used in official NOAA/National Weather Service heat advisories and warnings issued across the United States. Because heat index only accounts for shade-level conditions, being in direct sunlight can feel even hotter than the calculated value suggests, which is why NOAA advisories often add several extra degrees of caution for full sun exposure.
On a hot day, two locations at the exact same air temperature can feel completely different depending on how humid the air is. That perceived difference is what the heat index measures, and it's a far more useful number for judging real-world heat risk than air temperature alone — which is why it's the figure behind official heat advisories rather than the raw thermometer reading.
The reason humidity matters so much comes down to how the human body cools itself. Sweating only cools you effectively when that sweat can evaporate, carrying heat away from your skin. In humid air, the surrounding atmosphere is already closer to saturated with moisture, which slows evaporation dramatically — so your body's main cooling mechanism becomes less effective at exactly the moment you need it most. The result is that 95°F at high humidity can feel, and functionally behave, more like well over 100°F to your body.
The formula this calculator uses comes directly from the U.S. National Weather Service, based on a regression equation developed from a more complex underlying biometeorological model of apparent temperature. NOAA also publishes small correction adjustments for edge cases — for instance, unusually low humidity paired with high heat, or unusually high humidity at more moderate hot temperatures — both of which this tool applies automatically so the result matches official calculations rather than a simplified approximation.
Understanding your heat index reading matters for more than comfort. The National Weather Service ties specific health-risk categories to heat index ranges: caution in the 80-90°F range, extreme caution from 91-103°F, danger from 103-124°F, and extreme danger above 125°F, with risks like heat cramps, heat exhaustion, and heat stroke becoming more likely at each higher level, especially with continued outdoor activity or physical exertion. Checking the heat index rather than just the air temperature before spending extended time outdoors in summer is one of the simplest ways to gauge real heat risk for yourself, children, older adults, and anyone engaged in physical activity or work outdoors.
Because the human body relies heavily on sweat evaporation to cool itself, and high humidity slows that evaporation significantly, trapping more heat near the skin and making the same air temperature feel noticeably hotter than it would in dry conditions.
The National Weather Service generally considers a heat index of 80-90°F as requiring caution, 91-103°F as extreme caution, 103-124°F as danger, and 125°F+ as extreme danger, with health risk increasing at each level, especially with prolonged exposure or physical activity.
No — the heat index is specifically designed for warm-to-hot conditions; for cold and windy conditions, use our separate Wind Chill Calculator instead, which uses an entirely different formula suited to cold weather.
They measure a similar concept — how hot humid air feels — but are calculated with different formulas and are not numerically interchangeable; this tool specifically implements the NOAA heat index used in the United States.