Heat Index Calculator
Calculate the heat index using air temperature and relative humidity. See the estimated feels-like temperature instantly in both Fahrenheit and Celsius.
Calculate Heat Index
Enter the air temperature and relative humidity to calculate the estimated heat index.
What Is a Heat Index Calculator?
A Heat Index Calculator estimates how hot conditions may feel when air temperature and relative humidity are considered together. Heat index is sometimes described as an apparent or feels-like temperature.
High humidity can reduce the rate at which perspiration evaporates from the skin. Because evaporation is an important cooling process, hot and humid conditions can feel different from the measured air temperature alone.
How to Use the Heat Index Calculator
You only need the air temperature and relative humidity to use this calculator. The result is automatically displayed in Fahrenheit and Celsius.
Enter Air Temperature
Enter the measured air temperature and select either Fahrenheit or Celsius.
Enter Relative Humidity
Use the humidity slider or enter a percentage directly into the humidity field.
Calculate Heat Index
Click the calculation button to see the estimated feels-like temperature in both °F and °C.
How Is Heat Index Calculated?
This calculator uses the commonly used Rothfusz regression for sufficiently hot conditions. The equation estimates heat index from temperature in Fahrenheit and relative humidity.
HI = -42.379 + 2.04901523T + 10.14333127RH − 0.22475541TRH − 0.00683783T² − 0.05481717RH² + 0.00122874T²RH + 0.00085282TRH² − 0.00000199T²RH²
In the formula, T represents air temperature in degrees Fahrenheit and RH represents relative humidity as a percentage. For conditions outside the main range of the regression, this calculator uses a simpler preliminary apparent-temperature estimate.
Heat Index Calculation Example
Suppose the air temperature is 90°F and relative humidity is 60%. The measured air temperature alone does not fully describe how the combination of heat and humidity may feel.
Using the heat-index equation gives an apparent temperature of approximately 100°F, although rounding can vary slightly depending on the calculation method and adjustments used.
This demonstrates why humidity becomes increasingly important when evaluating hot-weather conditions.
Heat Index vs. Air Temperature
| Measurement | What It Represents | Inputs |
|---|---|---|
| Air Temperature | Measured temperature of the surrounding air | Temperature measurement |
| Heat Index | Estimated apparent temperature under specified assumptions | Temperature + humidity |
| Dew Point | Temperature associated with air reaching saturation | Temperature + humidity |
What Affects How Hot It Feels?
Air Temperature
Air temperature is the main input. Heat-index calculations become particularly relevant during hot conditions.
Relative Humidity
Humidity affects evaporation. When humidity is high, perspiration can evaporate less efficiently, which can increase the apparent heat under hot conditions.
Direct Sunlight
Standard heat-index calculations are generally associated with shaded conditions. Direct solar exposure can make outdoor conditions feel hotter than the calculated value.
Wind and Air Movement
Air movement can affect evaporation and thermal comfort. The standard heat-index formula does not fully represent every wind condition.
Activity and Clothing
Physical activity, clothing, hydration, health, and individual characteristics can all affect how a person experiences hot conditions. A calculated heat index should therefore be treated as an environmental estimate rather than an exact personal response.
Understanding Your Heat Index Result
As heat index rises, environmental heat stress can become more significant. However, the number should be interpreted alongside actual conditions, including sun exposure, activity, airflow, and duration outdoors.
| Heat Index | General Category |
|---|---|
| Below 80°F / 27°C | Below the commonly emphasized heat-index range |
| 80–89°F / 27–32°C | Increasing heat stress |
| 90–102°F / 32–39°C | High heat stress |
| 103–124°F / 39–51°C | Very high heat stress |
| 125°F / 52°C and above | Extreme heat conditions |
Heat Index Calculator FAQs
Heat index is an apparent-temperature estimate that combines air temperature and relative humidity to describe how hot conditions may feel under the calculation's assumptions.
Heat index can be calculated from air temperature and relative humidity using an empirical equation. This calculator uses the Rothfusz regression for sufficiently hot conditions.
Higher humidity can slow the evaporation of perspiration. Because evaporation helps remove heat from the body, hot and humid air can create a higher apparent temperature.
Yes. Select Celsius and enter the air temperature. The calculator handles the required unit conversion automatically and shows results in both Celsius and Fahrenheit.
No. Air temperature is a measured physical quantity, while heat index is an estimated apparent temperature calculated from air temperature and humidity.
Standard heat-index values are generally associated with shaded conditions. Direct sunlight can increase the heat experienced by a person beyond the standard calculated value.