Heat Index Calculator
0°F heat index
| Heat Index Range | Category | General Effect |
|---|---|---|
| 80–90°F | Caution | Fatigue possible with prolonged exposure or activity |
| 90–103°F | Extreme Caution | Heat cramps or heat exhaustion possible |
| 103–124°F | Danger | Heat exhaustion likely; heat stroke possible |
| 125°F or higher | Extreme Danger | Heat stroke highly likely |
The Rothfusz Regression, NWS's Official Formula
The National Weather Service computes heat index with the Rothfusz regression, a polynomial fit to Steadman's original heat-stress model: HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T² - 0.05481717R² + 0.00122874T²R + 0.00085282TR² - 0.00000199T²R², where T is air temperature in °F and R is relative humidity as a percentage. This regression is only valid, and only used by this calculator, when the full-formula result comes out at 80°F or higher; below that threshold a simpler average-based formula (0.5 × [T + 61 + (T-68)×1.2 + R×0.094]) is used instead, matching NWS practice for milder conditions.
Two Correction Terms for Extreme Humidity
The base regression alone drifts noticeably at the humidity extremes, so NWS applies two adjustments before finalizing the figure. When relative humidity is below 13% and temperature falls between 80°F and 112°F, a correction of [(13-R)/4] × √[(17-|T-95|)/17] is subtracted, reflecting how dry air lets sweat evaporate faster than the base formula assumes. When relative humidity exceeds 85% and temperature is between 80°F and 87°F, a correction of [(R-85)/10] × [(87-T)/5] is added, since very humid air suppresses evaporative cooling more than the base terms capture. This calculator applies whichever correction matches your inputs automatically.
Heat Index vs. Actual Temperature
Heat index (sometimes called the "feels like" temperature) estimates how hot it actually feels to the human body by accounting for humidity's effect on evaporative cooling — it assumes shade and light wind, so direct sun exposure can push the real effect several degrees higher. It's the summer counterpart to the wind chill calculator, which does the equivalent adjustment for cold, windy conditions. If you need to convert the result to Celsius or another unit, the conversion calculator can handle that.
Frequently Asked Questions
Why does the calculator sometimes show a different result than a simple online chart?
Heat index charts are usually rounded lookup tables built from the same NWS Rothfusz regression this calculator computes directly, so small differences (often under 1°F) come from chart rounding, not a different method. This calculator also applies the official low-humidity and high-humidity correction terms that many simplified charts omit.
Why does the heat index formula only apply above 80°F?
The Rothfusz regression was fit to Steadman's heat-stress data specifically for warm, humid conditions and becomes unreliable below about 80°F. Below that threshold, this calculator automatically switches to a simpler averaging formula, matching how the National Weather Service itself handles milder temperatures.