Heat Index Calculator
Calculate the official NOAA/NWS Heat Index ("feels like" temperature) from air temperature and relative humidity.
How It's Calculated
Formula
HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T^2 - 0.05481717R^2 + 0.00122874T^2R + 0.00085282TR^2 - 0.00000199T^2R^2This 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.