About this tool
Heat Index (Feels-Like) Calculator is a free, in-browser tool that estimates how hot it actually feels when humidity is factored into the air temperature. Enter the temperature and relative humidity, choose °F or °C, and it returns the apparent temperature plus a heat-risk category. All maths runs locally; the values are yours, not live weather.
The calculation works in Fahrenheit (Celsius inputs are converted first). Below 80 °F it uses the simple form HI = 0.5·(T + 61 + (T − 68)·1.2 + RH·0.094). At or above 80 °F it uses the full NWS Rothfusz regression with terms in T and RH, plus two documented corrections: a subtraction when humidity is under 13% in hot weather, and an addition when humidity is above 85% at the low-80s. The result is converted back to °C if you chose it.
Heat-risk bands follow the National Weather Service: Caution 80–90 °F, Extreme Caution 90–103 °F, Danger 103–124 °F and Extreme Danger 125 °F and above. Because both inputs are user-entered, you can test any scenario without connecting to a weather feed.