Heat Index Calculation
Estimate apparent temperature using air temperature and relative humidity.
Weather Heat Index Calculator
Calculate the perceived outdoor temperature by combining air temperature and relative humidity. Use the heat index to better understand hot-weather conditions and plan outdoor activities more safely.
Weather Details
Enter temperature and humidity
°F
Heat index is most useful for hot-weather conditions.
%
Heat Index
Perceived outdoor temperature
°
Feels Like
— °F
Enter weather details
Your perceived temperature will appear here.
Actual Temperature
—
Humidity
—
Heat Index °C
—
Feels Like Difference
—
Heat Safety
Heat index guidance will appear after calculation.
FREE ONLINE WEATHER HEAT INDEX CALCULATOR
Weather Heat Index Calculator
Calculate How Hot It Really Feels
Calculate the heat index using air temperature and relative humidity to estimate how hot conditions may feel to the human body.
Useful for checking hot-weather conditions, outdoor planning, travel, exercise, work activities, and understanding the effect of humidity on perceived heat.
Free to Use
Temperature + Humidity
Heat Index Estimate
Browser Based
Quick Tool Overview
The All Free Tool's Weather Heat Index Calculator estimates the apparent temperature using air temperature and relative humidity.
Enter the current air temperature and relative humidity to calculate an estimated heat index and understand how hot the conditions may feel.
Heat index is an estimate and should not replace local weather alerts, official heat advisories, or guidance from relevant weather authorities.
About the Weather Heat Index Calculator
What Is the Heat Index?
The heat index is an estimate of how hot it feels when the effects of air temperature and humidity are combined. Higher humidity can make hot conditions feel warmer because it reduces the body's ability to cool itself through evaporation.
Why Does Humidity Affect How Hot It Feels?
When humidity is high, sweat evaporates less efficiently. Because evaporation helps remove heat from the body, humid conditions can make the same air temperature feel considerably hotter.
Temperature and Relative Humidity
Heat index calculations use air temperature together with relative humidity. The combination of these measurements provides an estimate of apparent heat under suitable environmental conditions.
How the Weather Heat Index Calculator Works
Enter the air temperature and relative humidity into the calculator. The tool applies the appropriate heat index calculation and displays the resulting apparent temperature estimate.
Heat Index Is Different From Air Temperature
Air temperature measures the temperature of the air, while heat index attempts to describe how hot the environment may feel under the combined influence of temperature and humidity.
When Heat Index Becomes More Important
The heat index becomes particularly useful during hot and humid weather. As temperatures and humidity increase, the apparent heat can become significantly greater than the measured air temperature.
Outdoor Activities
Heat index information can help people think about outdoor conditions when planning activities such as walking, sports, exercise, travel, construction, gardening, or other physical work.
Heat Index and Weather Safety
A high heat index can indicate increased heat stress risk. During very hot conditions, follow local weather advisories and take appropriate precautions such as staying hydrated, taking breaks, seeking shade or cooling, and limiting strenuous activity when advised.
Heat Index Is an Estimate
Heat index values are estimates based on specific environmental assumptions. Actual perceived heat can vary depending on sunlight, wind, clothing, physical activity, acclimatization, and individual conditions.
Key Features
Temperature Input
Enter the measured air temperature for the calculation.
Humidity Input
Enter relative humidity to account for the effect of moisture in the air.
Apparent Temperature
View an estimated temperature representing how hot the conditions may feel.
Humidity Effect
Understand how relative humidity can increase perceived heat during hot weather.
Outdoor Planning
Use heat index estimates as one factor when planning outdoor activities.
Easy-to-Understand Results
Present the calculated heat index in a clear format that is easy to interpret.
Quick Calculation
Calculate the estimated heat index without manually applying the formula.
Weather Awareness
Use the result to better understand the combined effect of heat and humidity.
Browser Based
Calculate heat index directly from your browser without installing additional software.
How to Use the Weather Heat Index Calculator
Follow these simple steps to calculate an estimated heat index from temperature and humidity.
Check the Air Temperature
Obtain the current air temperature from a reliable thermometer or weather source.
Pro Tip: Use a current temperature reading that represents the location you are evaluating.
Enter the Temperature
Enter the measured air temperature into the calculator using the supported temperature unit.
Pro Tip: Double-check whether your temperature is entered in Celsius or Fahrenheit.
Check Relative Humidity
Find the relative humidity for the same location and approximate time.
Pro Tip: Temperature and humidity should represent the same conditions for a useful estimate.
Enter Relative Humidity
Enter the relative humidity percentage into the calculator.
Pro Tip: Enter humidity as a percentage, such as 60%, rather than as a decimal.
Calculate the Heat Index
Start the calculation to estimate the apparent temperature from the supplied conditions.
Pro Tip: Use the calculator result as an estimate rather than an exact measurement.
Review the Result
Review the estimated heat index and compare it with the actual air temperature.
Pro Tip: A higher heat index means the conditions may feel hotter than the measured temperature.
Consider the Conditions
Consider sunlight, wind, physical activity, clothing, and other environmental factors that may affect how hot conditions feel.
Pro Tip: Heat index does not capture every environmental factor that affects heat stress.
Follow Weather Advisories
For high-heat conditions, check official local weather alerts and follow recommended safety guidance.
Pro Tip: Do not rely on a calculator alone when official heat warnings or advisories are issued.