Using the Heat Index Calculator
Enter air temperature in Fahrenheit within the supported range and relative humidity as a percentage. The tool reports the calculated index in both Fahrenheit and Celsius. Those outputs are two unit representations of one estimate, not separate forecasts.
Worked example
Use the example inputs below to reproduce this result. These are the form's starting values, not recommended targets. Changing your inputs updates your result above; this worked example stays fixed for comparison. Results are rounded for display.
Example inputs
- Air temperature (°F)
- 90
- Relative humidity (%)
- 60
Example result
- Heat index (°F)
- 99.677718
- Heat index (°C)
- 37.598732
Method and assumptions
NWS Rothfusz regression with low/high humidity corrections, for shaded light-wind conditions. Restricted to 80–112°F; sun exposure can increase heat stress.
Reference: weather.gov — method and further reading
Use temperature and humidity from the same conditions
Temperature and relative humidity should describe the same place and time. Combining an indoor temperature with an outdoor humidity reading can produce a number that does not represent either location. The model explains the entered conditions; it does not retrieve observations, forecast changes or replace local weather guidance.
Common question
Is heat index the same as the thermometer temperature?
No. A thermometer reports air temperature, while heat index combines temperature and humidity to estimate apparent heat under specified conditions. It should not replace local warnings or be read as an exact description of how every person will feel.
Guide updated .