Heat Index Calculator

Calculate apparent temperature and heat stress based on air temperature and humidity.

Determine how hot it actually feels by combining air temperature and relative humidity. This helps assess heat-related health risks and plan outdoor activities safely.

Examples

Click on any example to load it into the calculator.

Moderate Heat Conditions

Moderate Heat Conditions

Typical summer day with moderate heat and humidity.

Temperature: 85 °F

Humidity: 70 %

Temp (°C): 29.4 °C

High Heat Stress

High Heat Stress

Hot and humid conditions requiring caution.

Temperature: 95 °F

Humidity: 80 %

Temp (°C): 35 °C

Extreme Heat Warning

Extreme Heat Warning

Dangerous heat conditions requiring extreme caution.

Temperature: 105 °F

Humidity: 75 %

Temp (°C): 40.6 °C

Comfortable Conditions

Comfortable Conditions

Pleasant weather with low heat stress.

Temperature: 75 °F

Humidity: 50 %

Temp (°C): 23.9 °C

Other Titles
Understanding Heat Index Calculator: A Comprehensive Guide
Explore the science behind heat index calculations and learn how to interpret apparent temperature for better heat safety and comfort planning.

What is Heat Index?

  • Core Concepts
  • Why It Matters
  • Heat Index vs. Actual Temperature
Heat index, also known as apparent temperature, is a measure that combines air temperature and relative humidity to determine how hot it actually feels to the human body. Unlike the actual air temperature, heat index accounts for the body's reduced ability to cool itself through perspiration when humidity is high. This makes it a crucial metric for understanding heat-related health risks and planning outdoor activities safely.
The Science Behind Heat Index
The human body maintains its core temperature through various cooling mechanisms, primarily sweating and evaporation. When the air is humid, sweat doesn't evaporate as efficiently, reducing the body's natural cooling ability. The heat index quantifies this effect by calculating the apparent temperature that would produce the same level of heat stress under standard conditions. This calculation is based on extensive research and physiological studies conducted by the National Weather Service.
Why Heat Index Matters More Than Temperature Alone
A temperature of 90°F (32°C) with 20% humidity feels much different than the same temperature with 80% humidity. The heat index helps us understand this difference by providing a single number that represents the combined effect of temperature and humidity on human comfort and health. This is especially important for vulnerable populations like the elderly, children, and those with pre-existing health conditions.
Heat Index vs. Wind Chill
While heat index measures how hot it feels due to high humidity, wind chill measures how cold it feels due to wind speed. Both are apparent temperature measures, but they work in opposite directions. Heat index is most relevant during warm months, while wind chill is important during cold weather. Understanding both helps in comprehensive weather planning and safety.

Key Heat Index Categories:

  • Caution (80-90°F): Fatigue possible with prolonged exposure
  • Extreme Caution (90-103°F): Heat cramps and exhaustion possible
  • Danger (103-124°F): Heat cramps, exhaustion, and heat stroke likely
  • Extreme Danger (125°F+): Heat stroke highly likely

Step-by-Step Guide to Using the Calculator

  • Gathering Data
  • Inputting Values
  • Interpreting Results
Using the heat index calculator is straightforward, but understanding how to gather accurate data and interpret the results is crucial for making informed decisions about heat safety.
1. Obtaining Accurate Temperature Data
Use a reliable thermometer placed in the shade, away from direct sunlight and heat sources. Digital thermometers are generally more accurate than analog ones. For outdoor measurements, place the thermometer at least 5 feet above the ground and away from buildings or pavement that might radiate heat. If you're using weather service data, ensure it's from a nearby, properly maintained weather station.
2. Measuring Relative Humidity
Relative humidity can be measured using a hygrometer or obtained from weather services. Humidity levels can vary significantly throughout the day, typically being highest in the early morning and lowest in the afternoon. For the most accurate heat index calculation, use humidity measurements taken at the same time as your temperature reading.
3. Entering Data and Understanding Units
The calculator accepts both Fahrenheit and Celsius temperature inputs. Choose the unit system you're most comfortable with, but ensure consistency. Enter the temperature and humidity values exactly as measured, without rounding. The calculator will automatically convert between units if needed and provide results in both Fahrenheit and Celsius.
4. Interpreting the Results
The calculator provides several key outputs: the heat index value, apparent temperature, heat stress level, and risk category. The heat stress level indicates the severity of heat-related health risks, while the risk category provides specific recommendations for outdoor activities and safety measures. Use these results to make informed decisions about outdoor work, exercise, and recreation.

Heat Index Safety Guidelines:

  • Below 80°F: Generally safe for all activities
  • 80-90°F: Limit strenuous outdoor activities
  • 90-103°F: Reduce outdoor activities, take frequent breaks
  • 103-124°F: Avoid outdoor activities during peak hours
  • 125°F+: Outdoor activities should be postponed

Real-World Applications and Heat Safety

  • Outdoor Activities
  • Workplace Safety
  • Public Health
Heat index calculations have numerous practical applications in daily life, from personal safety to public health policy and workplace regulations.
Outdoor Recreation and Sports
Athletic organizations, schools, and recreational facilities use heat index data to make decisions about outdoor activities. Many sports leagues have specific protocols for heat index thresholds, automatically postponing or modifying events when conditions become dangerous. Coaches and event organizers can use this calculator to assess conditions and implement appropriate safety measures, such as additional water breaks, reduced practice intensity, or schedule adjustments.
Workplace Safety and Regulations
Occupational safety regulations often include heat index thresholds for outdoor work. Construction, agriculture, and other outdoor industries must monitor heat conditions and implement protective measures when heat index values exceed certain levels. This calculator helps employers and safety managers make data-driven decisions about work schedules, protective equipment, and emergency procedures.
Public Health and Emergency Management
Public health officials use heat index data to issue heat advisories and warnings, coordinate emergency responses, and develop community cooling strategies. During heat waves, accurate heat index calculations help emergency managers allocate resources, open cooling centers, and communicate risks to vulnerable populations. This information is crucial for preventing heat-related illnesses and deaths.

Common Misconceptions and Heat Safety Myths

  • Temperature vs. Heat Index
  • Hydration Myths
  • Cooling Strategies
Many people have misconceptions about heat safety and how to interpret heat index values. Understanding these myths helps in making better decisions about heat exposure and safety measures.
Myth: If the Temperature is Below 100°F, It's Safe
This is one of the most dangerous misconceptions. A temperature of 85°F with 90% humidity can result in a heat index of 101°F, creating dangerous conditions even though the actual temperature seems moderate. The heat index, not the temperature alone, determines the level of heat stress and health risk. Always consider both temperature and humidity when assessing heat safety.
Myth: Drinking Cold Water is the Best Way to Cool Down
While staying hydrated is crucial, the temperature of the water matters less than the amount. Room temperature water is actually absorbed more quickly by the body than ice-cold water. The key is to drink enough fluids regularly, not just when you feel thirsty. Additionally, sports drinks with electrolytes can be beneficial for prolonged outdoor activities.
Myth: Fans are Always Effective for Cooling
Fans work by increasing air movement, which enhances evaporative cooling from sweat. However, when the heat index is very high (above 95°F), fans may actually increase heat stress by blowing hot air over the body. In extreme heat, air conditioning or other cooling methods are more effective than fans alone.

Effective Heat Safety Strategies:

  • Stay hydrated with water and electrolyte drinks
  • Wear lightweight, light-colored, loose-fitting clothing
  • Take frequent breaks in air-conditioned or shaded areas
  • Avoid strenuous activities during peak heat hours (10 AM - 4 PM)
  • Monitor weather forecasts and heat index values regularly

Mathematical Derivation and Calculation Methods

  • Steadman's Formula
  • National Weather Service Method
  • Limitations and Assumptions
The heat index calculation is based on complex mathematical formulas developed through extensive research and physiological studies. Understanding the underlying mathematics helps in appreciating the accuracy and limitations of heat index calculations.
Steadman's Heat Index Formula
The modern heat index calculation is based on the work of Robert G. Steadman, who developed a comprehensive formula that considers multiple factors affecting human thermal comfort. The formula includes polynomial terms that account for the non-linear relationship between temperature, humidity, and perceived heat. The calculation involves multiple steps and coefficients that vary based on the temperature range.
National Weather Service Implementation
The National Weather Service uses a simplified version of Steadman's formula that provides accurate results for most practical applications. This implementation includes temperature and humidity thresholds, beyond which the heat index calculation becomes less reliable. The NWS also provides adjustment factors for different conditions, such as full sun exposure or light wind.
Limitations and Assumptions
Heat index calculations assume standard conditions: light wind, full sun exposure, and average human physiology. Factors like wind speed, clothing, physical activity level, and individual health conditions can significantly affect actual heat stress. The calculations are most accurate for temperatures between 80°F and 110°F and humidity levels between 40% and 100%. Outside these ranges, the heat index may not accurately reflect actual heat stress.
Alternative Heat Stress Indices
While heat index is widely used in the United States, other countries and organizations use different heat stress indices. The Wet Bulb Globe Temperature (WBGT) considers additional factors like solar radiation and wind speed, making it more comprehensive for certain applications. The Universal Thermal Climate Index (UTCI) provides a more sophisticated analysis of thermal comfort under various environmental conditions.

Heat Index Calculation Example:

  • Temperature: 90°F, Humidity: 70% → Heat Index: 106°F
  • Temperature: 85°F, Humidity: 90% → Heat Index: 101°F
  • Temperature: 95°F, Humidity: 60% → Heat Index: 110°F
  • Temperature: 100°F, Humidity: 50% → Heat Index: 118°F