Heat Index Calculator
The heat index, sometimes called the apparent temperature or "feels like" temperature, tells you how hot the air actually feels when humidity is factored in alongside the measured air temperature. On a hot, humid summer day the temperature reading alone understates the risk to your health and comfort. Your body cools itself by evaporating sweat from your skin, but when relative humidity is high, that evaporation slows down and your body retains more heat. The National Weather Service uses the Rothfusz regression formula to compute the heat index whenever the air temperature is at or above 80°F and relative humidity is at or above 40%. Below those thresholds the formula gives unreliable results, so a simpler estimate applies. The NWS assigns four danger categories based on the heat index value: Caution (80-90°F), Extreme Caution (91-103°F), Danger (103-124°F), and Extreme Danger (125°F and above). Spending prolonged periods outdoors or exercising in Danger or Extreme Danger conditions without adequate hydration and rest can lead to heat exhaustion or heat stroke. Enter the current air temperature in degrees Fahrenheit and the relative humidity percentage to find out the apparent temperature and its danger category.
Heat index: -- °F (-- °C)
NWS heat index danger categories
| Heat index (°F) | Category | Possible effects |
|---|---|---|
| 80-90°F | Caution | Fatigue possible with prolonged exposure and activity |
| 91-103°F | Extreme Caution | Heat cramps and heat exhaustion possible |
| 103-124°F | Danger | Heat cramps and exhaustion likely; heat stroke possible |
| 125°F and above | Extreme Danger | Heat stroke highly likely |
Source: NOAA National Weather Service.
About the Rothfusz regression equation
The Rothfusz equation is a multiple-variable regression formula that best fits the Steadman (1979) apparent temperature tables. It requires both temperature (T, in °F) and relative humidity (RH, as a percentage). The equation is valid for temperatures at or above 80°F and relative humidity at or above 40%. NOAA applies adjustment terms for conditions with low humidity or very high temperatures.
For conditions below 80°F or below 40% relative humidity, a simpler linear approximation is used: HI = 0.5 * (T + 61.0 + (T-68.0)*1.2 + RH*0.094). This calculator automatically selects the appropriate method based on your inputs.
Heat index: frequently asked questions
What is the heat index?
The heat index is the apparent or "feels like" temperature that combines air temperature and relative humidity to reflect how hot conditions actually feel to the human body. High humidity reduces the body's ability to cool itself through sweat evaporation, making the air feel hotter than the thermometer reading.
What formula does this calculator use?
This calculator uses the Rothfusz regression equation published by NOAA's National Weather Service. It is valid when the air temperature is at or above 80°F and relative humidity is at or above 40%. Outside those bounds the simple Steadman formula is used, which gives a rough estimate.
When is the heat index dangerous?
The NWS classifies heat index values above 103°F as Danger, and above 125°F as Extreme Danger. Values between 91°F and 103°F are Caution or Extreme Caution. Prolonged exposure or physical activity at these levels can lead to heat exhaustion or heat stroke.
Why does humidity make it feel hotter?
The human body cools itself primarily by sweating. When the air is very humid, sweat evaporates more slowly, so less heat is removed from the body. The result is a higher effective or perceived temperature.
Can I use this for Celsius inputs?
The NOAA formula requires Fahrenheit inputs. This calculator accepts Fahrenheit and converts the result to Celsius for display. To convert Celsius to Fahrenheit, multiply by 9/5 and add 32.
References
- NOAA National Weather Service. "Heat Index." wpc.ncep.noaa.gov
- Rothfusz, L.P. "The Heat Index 'Equation' (or, More Than You Ever Wanted to Know About Heat Index)." NWS Technical Attachment SR/SSD 90-23, 1990.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.