Heat Index Calculator (Feels Like Temperature)

Enter your air temperature and relative humidity to calculate the apparent heat index, warning level, dew point, and how much humidity amplifies the heat.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter Air Temperature (°F)
Input the current air temperature in degrees Fahrenheit, e.g., '90'°F. The NWS formula is most accurate between 80-110°F.
2
Enter Relative Humidity (%)
Provide the relative humidity as a percentage (0-100), for example, '65'%. Higher humidity significantly raises the 'feels like' temperature.
3
Review Heat Index
The calculator will display the apparent 'feels like' temperature, risk level, dew point, and heat amplification.

Example Calculation

A person checking the 'feels like' temperature before engaging in outdoor activities on a hot and humid day.

Air Temperature (°F)

Relative Humidity (%)

Results

105.0 °F

Tips

Stay Hydrated

When the heat index is high (e.g., above 90°F), increase your fluid intake significantly. Drink water or electrolyte-rich beverages even if you don't feel thirsty, aiming for at least 8-10 glasses per day to prevent dehydration.

Limit Outdoor Activity

During periods of high heat index (e.g., Danger or Extreme Danger levels), reschedule strenuous outdoor activities to cooler parts of the day, such as early morning or late evening. Avoid prolonged exposure, especially between 10 AM and 4 PM.

Recognize Symptoms of Heat Illness

Be aware of symptoms like heavy sweating, weakness, dizziness, headache, nausea, and muscle cramps, which indicate heat exhaustion (Heat Index above 90°F). If core body temperature reaches 104°F (40°C), heat stroke can occur, a medical emergency requiring immediate attention.

The Heat Index Calculator provides the "feels like" temperature, a crucial metric for understanding real-world thermal comfort and safety. Using the National Weather Service (NWS) formula, it combines air temperature and relative humidity to assess the actual physiological stress on the human body. For instance, an air temperature of 90°F with 65% humidity results in a heat index of 105.0°F, classified as "Danger," warning individuals to take precautions against heat-related illnesses in 2025.

Public Health Warnings for High Heat Index

High heat index values trigger critical public health warnings from meteorological and health authorities, as they directly correlate with increased risks of heat-related illnesses. The National Weather Service (NWS) defines several warning levels: "Caution" for heat indices between 80-90°F (fatigue possible), "Extreme Caution" for 90-105°F (heat cramps and exhaustion possible), "Danger" for 105-130°F (heat cramps and exhaustion likely, heat stroke possible), and "Extreme Danger" for values above 130°F (heat stroke highly likely). These warnings, often issued as Heat Advisories or Excessive Heat Warnings, urge the public to limit outdoor activity, stay hydrated, and check on vulnerable individuals. The Centers for Disease Control and Prevention (CDC) emphasizes that heatstroke, a medical emergency where core body temperature exceeds 104°F (40°C), can be fatal if not treated immediately.

The NWS Heat Index Formula Explained

The Heat Index Calculator utilizes the National Weather Service (NWS) formula, a complex regression equation designed to accurately model the apparent temperature felt by the human body by combining air temperature and relative humidity. This formula is particularly sophisticated because it accounts for the non-linear interaction between these two variables, especially at higher temperatures where humidity's impact on cooling is amplified.

For temperatures at or above 80°F, the primary formula involves multiple coefficients:

HI = -42.379 + 2.04901523*T + 10.14333127*R - 0.22475541*T*R - 0.00683783*T*T - 0.05481717*R*R + 0.00122874*T*T*R + 0.00085282*T*R*R - 0.00000199*T*T*R*R

Where T is temperature in °F and R is relative humidity as a percentage. Specific adjustments are also applied for very low humidity (below 13%) and very high humidity (above 85%) within certain temperature ranges to refine the accuracy. For temperatures below 80°F, a simpler, linear approximation is used.

💡 Understanding the heat index helps plan for safe outdoor activities. Similarly, if you're assessing other environmental factors for travel, our Travel Weather Disruption Risk Calculator can help you anticipate potential delays or changes due to adverse conditions.

Calculating the Feels Like Temperature for a Hot Day

Let's calculate the heat index for a day with an air temperature of 90°F and 65% relative humidity.

Input Air Temperature (T): 90°F.
Input Relative Humidity (R): 65%.
Apply NWS Heat Index Formula: Since T is >= 80°F, the complex formula is used.
- HI = -42.379 + 2.04901523*(90) + 10.14333127*(65) - 0.22475541*(90)*(65) - 0.00683783*(90)*(90) - 0.05481717*(65)*(65) + 0.00122874*(90)*(90)*(65) + 0.00085282*(90)*(65)*(65) - 0.00000199*(90)*(90)*(65)*(65)
- This calculation yields approximately 105.0°F.
Determine Warning Level: A heat index of 105°F falls into the "Danger" category, indicating that heat cramps and exhaustion are likely, and heat stroke is possible.
Calculate Heat Amplification: The heat index (105°F) minus the air temperature (90°F) is 15°F, showing a significant amplification due to humidity.

The primary output, Heat Index, is 105.0°F.

💡 Just as the heat index quantifies one environmental risk, other natural elements also affect safety. Our UV Index Calculator helps you assess your risk of sun exposure, providing another vital safety metric for outdoor planning.

Public Health Warnings for High Heat Index

Heat Index Thresholds and Risk Categories

The National Weather Service (NWS) utilizes specific heat index thresholds to define risk categories, providing critical guidance for public safety during hot and humid conditions. These benchmarks are crucial for understanding the potential for heat-related illnesses:

Caution (Heat Index 80°F-90°F): At this level, fatigue is possible with prolonged exposure and physical activity. Individuals should stay hydrated and take breaks.
Extreme Caution (Heat Index 90°F-105°F): Heat cramps and heat exhaustion are possible with prolonged exposure and physical activity. This is a common threshold for issuing heat advisories.
Danger (Heat Index 105°F-130°F): Heat cramps and heat exhaustion are likely, and heat stroke is possible with prolonged exposure. This level often triggers excessive heat warnings, urging significant precautions.
Extreme Danger (Heat Index > 130°F): Heat stroke is highly likely. This is a rare but extremely dangerous category, necessitating immediate and severe restrictions on outdoor activity and close monitoring for symptoms.

These thresholds are based on physiological responses to combined heat and humidity, directly informing public health recommendations and emergency preparedness strategies. For example, local authorities often activate cooling centers when the heat index consistently reaches the "Danger" category.

Frequently Asked Questions

What is the heat index?

The heat index, also known as the apparent temperature or 'feels like' temperature, is a measure that combines air temperature and relative humidity to determine how hot it actually feels to the human body. It quantifies the combined effect of heat and humidity on the human body's ability to cool itself through sweating. A higher heat index indicates a greater risk of heat-related illnesses, as the body struggles to dissipate heat effectively when the air is saturated with moisture.

How is the heat index calculated?

The heat index is calculated using a complex formula developed by the National Weather Service (NWS) that accounts for the non-linear interaction between air temperature and relative humidity. While there are simpler approximations, the NWS formula involves multiple regression equations and adjustments for specific temperature and humidity ranges. For temperatures below 80°F, a simpler formula is often used, but above this threshold, a more elaborate calculation provides a precise 'feels like' temperature, reflecting the body's actual thermal sensation.

What are the NWS heat index risk categories?

The NWS defines several heat index risk categories to guide public safety: 'Caution' (80-90°F) for fatigue possible; 'Extreme Caution' (90-105°F) for heat cramps/exhaustion possible; 'Danger' (105-130°F) for heat cramps/exhaustion likely, heat stroke possible; and 'Extreme Danger' (130°F+) for heat stroke highly likely. These categories emphasize the increasing health risks associated with higher apparent temperatures, prompting individuals to take necessary precautions like limiting outdoor activity and staying hydrated.

Does wind affect the heat index?

No, wind is not directly incorporated into the standard heat index calculation. The heat index specifically measures the combined effect of air temperature and relative humidity on perceived temperature. While wind can help cool the body through increased convection and evaporation of sweat, this effect is separate from the heat index. The NWS heat index formula focuses solely on the environmental factors that impede the body's natural cooling mechanisms, not on those that enhance it, which is why wind chill is a separate calculation for cold conditions.