The National Weather Service's heat index regression is fit to conditions at 80°F (27°C) and above with at least 40% humidity — the range where heat stress is actually a health concern. Below that, this calculator switches to a simpler approximation instead of stretching the full formula somewhere it was never validated.
Heat index formula, the NWS Rothfusz regression
Enter temperature and relative humidity and this calculator applies the official regression, including the extra correction terms NWS adds for very dry or very humid extremes within its valid range.
Feels like temperature humidity calculator, with real limitations disclosed
The formula assumes an average body size in the shade with light wind — actual perceived heat still varies by individual, clothing, activity, and direct sun exposure, and this calculator says so rather than implying false precision.
How hot does it feel with humidity, in °F or °C
Switch units to match your local forecast — the underlying regression is Fahrenheit-based, so °C inputs are converted before calculating and converted back for display.
