Heat Index Calculator
Calculate the official NOAA/NWS Heat Index ("feels like" temperature) from air temperature and relative humidity.
Calculator Inputs
Ambient air temperature, in degrees Fahrenheit.
Relative humidity, greater than 0 and up to 100 percent.
- Heat Index ("Feels Like")
- 94.60
- Method Used
- Rothfusz regression (NWS official)
About these results
CalcSpectrum calculators produce automated estimates using established formulas and the specific values you enter. Because real-world outcomes depend on input accuracy, mathematical assumptions, and rounding conventions, please verify critical figures with an appropriately qualified financial, tax, medical, legal, or other professional, as relevant.
How It's Calculated
Formula
This calculator uses the official NOAA/National Weather Service Heat Index methodology, based on Lans Rothfusz's 1990 regression of Steadman's apparent-temperature model. A simple initial estimate is always computed first; if the average of that estimate and the air temperature stays below 80°F, the simple estimate is used directly, since the full regression is only calibrated for hotter, more dangerous conditions. Otherwise, the full Rothfusz regression is applied, with two official correction terms: a downward adjustment for low humidity (below 13%) between 80-112°F, and an upward adjustment for high humidity (above 85%) between 80-87°F. This calculator reports only the numeric Heat Index — it does not classify results into named heat-danger categories.
Worked Examples
90°F, 50% relative humidity
- Simple estimate combined with T exceeds 80°F, so the Rothfusz regression is used
- Applying the regression with T=90, RH=50 gives Heat Index ≈ 94.6°F
75°F, 60% relative humidity (below threshold)
- Simple estimate = 0.5 x (75 + 61 + (75-68)(1.2) + 60(0.094)) ≈ 75.0°F
- Average of simple estimate and T stays below 80°F, so the simple estimate is the final Heat Index
Frequently Asked Questions
Why does this calculator sometimes skip the Rothfusz regression?
The Rothfusz regression is a curve fit calibrated for hot, humid conditions. NOAA's official method first computes a simpler estimate, and only switches to the full regression when the combined effect of temperature and that estimate reaches 80°F — below that, the simple estimate is more appropriate and is used directly.
Does this calculator show a heat danger classification?
No. This version reports only the numeric Heat Index temperature. Danger-stage classification (e.g. caution, extreme danger) is a separate concern kept out of this calculator's current scope.
Why are there separate corrections for low and high humidity?
NOAA's official Rothfusz-based algorithm includes two correction terms because the base regression alone can overstate or understate the Heat Index at very low humidity (below 13%) or very high humidity (above 85%) within specific temperature ranges — these corrections keep the result accurate in those edge cases.