What Is the Heat Index?
On July 15, 2026, the temperature in Phoenix reads 108 degrees Fahrenheit. But the relative humidity is 45%. Your body cannot cool itself effectively because sweat cannot evaporate fast enough. The heat index says it feels like 137 degrees. Two hours later, humidity drops to 20% and the heat index falls to 107, even though the actual temperature has not changed. Humidity makes the difference between an uncomfortable afternoon and a dangerous medical emergency.
The heat index, also known as apparent temperature, measures how hot the air actually feels to the human body when relative humidity is factored in with the actual air temperature. High humidity prevents sweat from evaporating, which is your body's primary cooling mechanism. When sweat cannot evaporate, your body retains heat, making you feel hotter and increasing the risk of heat-related illness.
According to a 2026 study published in Frontiers in Cardiovascular Medicine analyzing CDC WONDER data, heat-related cardiovascular mortality in the United States increased sharply from 2015 to 2024, with males showing a significant upward trend. The Western and Southern US bear the highest burden. The National Weather Service provides heat forecast tools and safety guidance based on heat index values.
What This Calculator Does
Enter the current air temperature and relative humidity to calculate the apparent temperature that accounts for how hot it actually feels on the human body. The calculator also provides a risk assessment for heat-related health issues.
Inputs Required
- Air Temperature: The actual temperature in Fahrenheit or Celsius
- Relative Humidity: The moisture content of the air as a percentage
Outputs Provided
- Heat Index (Apparent Temperature): How hot the air feels to the human body
- Feels Hotter By: The difference between actual and apparent temperature
- Risk Level: Safe, Caution, Extreme Caution, Danger, or Extreme Danger
How the Calculation Works
The heat index formula used by the US National Weather Service is a regression equation that combines temperature and relative humidity to estimate how the human body perceives heat. The formula is complex because the relationship is non-linear: the effect of humidity increases dramatically at higher temperatures.
HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T squared - 0.05481717R squared + 0.00122874T2R + 0.00085282TR2 - 0.00000199T2R2
where T = temperature (F), R = relative humidity (%)
This formula applies when temperature is 80 degrees Fahrenheit (27 degrees Celsius) or above and humidity is 40% or higher. Below these thresholds, the apparent temperature is approximately equal to the actual temperature. The NWS notes that heat index values were devised for shady, light wind conditions. Exposure to full sunshine can increase heat index values by up to 15 degrees Fahrenheit. For cold weather apparent temperature, use our Wind Chill Calculator.
How to Use the Calculator
- Select your preferred unit (Fahrenheit or Celsius)
- Enter the current outdoor air temperature
- Enter the relative humidity percentage
- View the heat index and risk assessment
Example Calculations
Example 1: Coach Martinez, a high school football coach in El Paso, checks conditions before August practice. The temperature is 95 degrees Fahrenheit with 60% humidity. The heat index calculates to 118 degrees, which falls in the Danger category. He moves practice to 7 AM when the temperature drops to 82 degrees and humidity rises to 50%, producing a heat index of 87 degrees (Safe). He also checks the Dew Point Calculator to assess moisture levels.
Example 2: Maria, a construction site supervisor in Houston, monitors conditions for her crew throughout the day. At noon, the temperature is 91 degrees with 75% humidity, producing a heat index of 112 degrees (Danger). She implements mandatory 30-minute rest breaks every hour and provides electrolyte drinks. By 4 PM, clouds roll in and humidity drops to 55% while temperature stays at 90 degrees, reducing the heat index to 99 degrees (Caution). She adjusts the work schedule accordingly.
Real World Scenarios
Outdoor Work Safety
OSHA recommends that employers implement heat illness prevention plans when the heat index reaches 91 degrees Fahrenheit. At 103 degrees, the risk becomes high and triggers mandatory water breaks every 15 minutes. At 115 degrees or above, non-essential outdoor work should be postponed. A landscaping crew in Dallas uses this calculator every morning to plan their schedule, starting early when the heat index is below 90 and shifting to indoor tasks by midday.
Elderly Care
A family in Atlanta caring for an 82-year-old parent uses the calculator to assess indoor conditions during a power outage. The thermostat shows 85 degrees but humidity is 70%, producing a heat index of 96 degrees. Since older adults are less able to regulate body temperature, this poses a significant risk. They move the parent to a cooling center. The CDC identifies adults over 65, children under 4, and people with chronic medical conditions as most vulnerable to heat-related illness.
Event Planning
An organizer planning an outdoor wedding in New Orleans in July checks the typical afternoon conditions: 92 degrees with 70% humidity, yielding a heat index of 112 degrees. This falls in the Danger category. She moves the ceremony to 5 PM when temperatures typically drop to 88 degrees with 65% humidity, giving a heat index of 99 degrees (Caution). She also arranges shade structures and water stations. For temperature unit conversions, she uses our Temperature Converter.
Common Mistakes to Avoid
- Assuming temperature alone tells the risk: A 90 degree day with 40% humidity feels much cooler than 90 degrees with 80% humidity. The heat index at 90 degrees and 80% humidity is 113 degrees, which is in the Danger category. Always consider humidity.
- Ignoring acclimatization: People who are not used to hot weather are at higher risk even at lower heat index values. The NWS notes that early-season heat is particularly dangerous because bodies have not yet adapted. A 95 degree heat index in April is more dangerous than the same value in August.
- Relying on wind to cool you: Unlike wind chill, wind does not significantly reduce the heat index. The NWS formula assumes light wind conditions. While a breeze can help evaporate sweat slightly, in very hot and humid conditions the air is already saturated and wind provides minimal relief.
- Forgetting about direct sunlight: The heat index formula assumes shade. Full sun exposure can increase the apparent temperature by up to 15 degrees. A heat index of 100 degrees in shade becomes 115 degrees in direct sun.
Limitations of This Calculator
The heat index formula is valid for temperatures of 80 degrees Fahrenheit (27 degrees Celsius) and above with humidity of 40% or higher. Below these thresholds, the calculator returns the actual temperature as the apparent temperature, which is an approximation. The formula assumes a person wearing trousers and a short-sleeved shirt, walking at 3.1 mph, in shade with light wind. It does not account for direct sunlight (which can add up to 15 degrees), heavy exertion, protective clothing, individual heat tolerance, age, or medical conditions. The NWS has developed an experimental HeatRisk product that integrates additional factors including duration of heat, overnight low temperatures, and CDC health data. For cold weather conditions, use the Wind Chill Calculator. For moisture analysis, try the Dew Point Calculator.
Authoritative Research and Resources
- NWS Heat Forecast Tools is the official National Weather Service resource for heat safety, including the heat index chart, HeatRisk experimental product, and guidance for heat watches, warnings, and advisories. The NWS is a division of NOAA.
- CDC: Extreme Heat provides public health guidance on preventing heat-related illness, identifying warning signs of heat exhaustion and heat stroke, and protecting vulnerable populations. The CDC identifies adults over 65, children under 4, and people with chronic conditions as highest risk.
- OSHA Heat Illness Prevention provides employer guidelines for protecting outdoor workers, including recommended water break schedules, acclimatization protocols, and heat illness prevention plans tied to heat index thresholds.