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The calculator uses the Rothfusz regression, the official formula the US National Weather Service uses for heat advisories: a nine-term polynomial in air temperature (T, in Fahrenheit) and relative humidity (RH, in percent) that reproduces how the human body perceives combined heat and humidity. Two official adjustments are applied: when humidity is below 13 percent and the temperature is between 80 and 112 degrees Fahrenheit, a small amount is subtracted; when humidity is above 85 percent and the temperature is between 80 and 87 degrees, a small amount is added. Below 80 degrees Fahrenheit the full regression is unstable, so the heat index is reported as approximately the air temperature, matching NWS practice. Celsius inputs are converted to Fahrenheit first and the result is converted back. Every result also shows how much hotter it feels than the thermometer and the NWS heat risk category. These are planning estimates, not professional medical advice; always follow official heat advisories.
The heat index formula is a multiple regression published by Lans P. Rothfusz of the National Weather Service, fitted to reproduce apparent temperatures from Steadman's human heat-balance model using only air temperature and relative humidity. The full equation is: HI = -42.379 + 2.04901523 × T + 10.14333127 × RH - 0.22475541 × T × RH - 0.00683783 × T² - 0.05481717 × RH² + 0.00122874 × T² × RH + 0.00085282 × T × RH² - 0.00000199 × T² × RH², where T is the air temperature in Fahrenheit and RH is the relative humidity in percent. The squared and cross terms make the result climb steeply when both heat and humidity are high, which mirrors how fast the body loses its ability to cool itself. Two NWS adjustments fine-tune the edges: a downward correction for very dry air (humidity below 13 percent, 80 to 112 degrees) and an upward correction for very humid air (humidity above 85 percent, 80 to 87 degrees). This calculator applies both.
Enter the air temperature and choose Fahrenheit or Celsius, then enter the relative humidity as a percentage from 0 to 100. Press Calculate to get the heat index feels-like temperature in your chosen unit, how much hotter it feels than the thermometer reading, and the NWS heat risk category from Comfortable to Extreme Danger. The formula is designed for temperatures around 80 degrees Fahrenheit (27 degrees Celsius) and above; below that, humidity barely changes how heat feels, so the result will read close to the air temperature. For the most accurate reading, measure temperature and humidity at the same place and time, in the shade.
The National Weather Service groups heat index values into bands that drive heat advisories. Below 80 degrees Fahrenheit (27 Celsius): Comfortable, with minimal risk for most people. From 80 to 90 degrees (27 to 32 Celsius): Caution, where fatigue is possible with prolonged exposure and activity. From 91 to 103 degrees (32 to 39 Celsius): Extreme Caution, where heat cramps and heat exhaustion are possible. From 104 to 124 degrees (39 to 51 Celsius): Danger, where heat exhaustion is likely and heatstroke is possible with continued activity. At 125 degrees (52 Celsius) and above: Extreme Danger, where heatstroke is highly likely. The NWS typically issues heat alerts when the index is expected to exceed 105 to 110 degrees for at least two consecutive days. To read the official chart yourself, find the temperature on the left, move right to the humidity column, and read the feels-like value at the intersection. Weather services sometimes call these bands the heat index danger levels.
Take an air temperature of 95 degrees Fahrenheit with 60 percent relative humidity. Plugging T = 95 and RH = 60 into the Rothfusz regression gives: -42.379 + 194.66 + 608.60 - 1281.11 - 61.71 - 197.34 + 665.36 + 291.66 - 64.66, which totals about 113.1 degrees Fahrenheit. The thermometer says 95, but the humidity pushes the apparent temperature 18.1 degrees higher, into the Danger band where heat exhaustion is likely. In Celsius terms, 35 degrees at 60 percent humidity feels like 45.1 degrees. Neither the low-humidity nor the high-humidity adjustment applies here, since 60 percent sits in the middle range.
When the heat index climbs into the Danger bands, a few habits sharply cut your risk. Drink more fluids than usual and do not wait until you feel thirsty; avoid alcohol, caffeine, and very sugary drinks, which dehydrate. Wear lightweight, light-colored, loose clothing plus a wide-brimmed hat and sunglasses, and wear sunscreen, because sunburn makes it harder for your body to shed heat. Stay in the shade or air conditioning, especially between 11 a.m. and 3 p.m., and cut back on strenuous exercise in the heat of the day. Never leave children, older adults, or pets in a parked vehicle. Learn the warning signs: heavy sweating, hot and dry skin, rapid pulse, pale or clammy skin, and cramps can signal heat disorder, and suspected heatstroke is an emergency.
The heat index and wind chill are the two official feels-like measures, covering opposite seasons: the heat index combines temperature with humidity for summer heat, while wind chill combines temperature with wind speed for winter cold. The heat index assumes shade and light wind; direct sunshine can add up to 15 degrees Fahrenheit to what you feel, and strong dry wind changes the picture too. Commercial RealFeel-style numbers from weather apps are different products: they blend temperature, humidity, wind, sun intensity, and sometimes elevation into one branded figure, while the NOAA heat index uses only temperature and humidity. For workplace heat rules, many countries use the wet-bulb globe temperature (WBGT) instead, which adds solar and radiant heat measurements. Try our wind chill calculator when winter arrives.
The heat index is a general-population estimate built on a reference person of average build in light clothing at moderate activity, so age, fitness, acclimatization, medications, chronic illness, pregnancy, hydration, and clothing all shift real risk. It assumes shady, light-wind conditions, so treat it as an underestimate in full sun. It is not an occupational exposure standard and not medical advice: for decisions about outdoor work, sports, or vulnerable people, follow the heat advisories issued by your national or regional weather service, which account for local climate and duration.
The heat index is calculated with the Rothfusz regression: HI = -42.379 + 2.04901523 × T + 10.14333127 × RH - 0.22475541 × T × RH - 0.00683783 × T squared - 0.05481717 × RH squared + 0.00122874 × T squared × RH + 0.00085282 × T × RH squared - 0.00000199 × T squared × RH squared, where T is the air temperature in Fahrenheit and RH is the relative humidity in percent. The NWS then applies a small downward adjustment for very dry air and a small upward adjustment for very humid air.
Take notice from 80 degrees Fahrenheit (Caution), where fatigue is possible with prolonged activity. From 91 degrees (Extreme Caution), heat cramps and heat exhaustion are possible. From 104 degrees (Danger), heat exhaustion is likely and heatstroke is possible. At 125 degrees and above (Extreme Danger), heatstroke is highly likely. The NWS issues alerts when the index is expected to top 105 to 110 degrees for two or more days.
The standard heat index assumes shady conditions with a light wind. Direct full sunshine can add up to about 15 degrees Fahrenheit to the perceived temperature, so treat the calculated value as an underestimate when you are in open sun. Strong wind also changes how heat feels, which is why workplace standards often use the wet-bulb globe temperature instead.
Your body cools itself mainly by evaporating sweat. When the air is already full of moisture, sweat evaporates slowly and your body retains more heat at the same air temperature. That is why a humid 88-degree day can feel as punishing as a dry 100-degree day, and why the heat index climbs steeply as humidity rises.
Not exactly. The NOAA heat index combines only air temperature and relative humidity into a feels-like temperature. Commercial RealFeel-type figures from weather apps also factor in wind speed, sun intensity, and sometimes elevation, so the two numbers often differ. For official heat advisories, the NWS uses the heat index.
The Rothfusz regression was designed for hot, humid conditions, roughly above 80 degrees Fahrenheit. At cooler temperatures or very low humidity it becomes less meaningful, and the result will simply read close to the actual air temperature, which is the correct interpretation. This calculator follows that NWS practice.
No. The heat index, risk category, and safety tips on this page are planning estimates based on the National Weather Service scale, not professional medical advice. Heat risk depends on age, health, medications, acclimatization, and clothing. Follow official heat advisories, and treat suspected heatstroke as an emergency requiring immediate medical help.
Air temperature is what a thermometer measures in the shade. The heat index adjusts that reading for humidity, showing how hot the air actually feels to your body because humid air slows the evaporation of sweat. The difference between the two numbers is the extra perceived heat added by humidity: at 95 degrees with 60 percent humidity, the difference is about 18 degrees.
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