Science & Engineering
Heat Index Calculator
Estimate the U.S. heat index from air temperature and relative humidity using the NOAA regression in its intended hot-weather range. The calculator uses NOAA Rothfusz regression using temperature °F and relative humidity %. It returns more than one result so you can check the main answer against a useful secondary measure. Heat index represents shaded, light-wind conditions and can be higher in direct sun. The regression is intended mainly for temperatures at or above 80°F and humidity at or above 40%; follow local warnings rather than this estimate.
Educational model only. Use applicable standards, calibrated measurements, and qualified review for safety-critical work.
Calculate and compare
Use the number box for precision or the slider for fast scenario testing.
Scenario results
Shade heat index
104.7 °F
NOAA regression estimate.
Adjusted comparison
104.7 °F
Includes the user-entered sun adjustment.
Difference from air temperature
12.7 °F
Apparent minus measured air temperature.
How the calculation works
Use consistent units and retain full precision until the final display step.
Worked example
Reproduce the displayed scenario, then change one assumption at a time.
Assumptions behind the result
- • Inputs use the units shown beside each control.
- • The displayed formula is applied without hidden market or demographic data.
- • Rounding occurs only for display; calculations keep full numeric precision.
- • Heat index represents shaded, light-wind conditions and can be higher in direct sun.
- • The regression is intended mainly for temperatures at or above 80°F and humidity at or above 40%; follow local warnings rather than this estimate.
Mistakes that change the answer
- • Mixing percentages with decimals or mixing incompatible units.
- • Relying on a rounded intermediate value instead of the full result.
- • Changing several assumptions at once instead of testing air temperature separately.