Heat index calculator
An illustrative heat index from the unadjusted regression.
Fill in the fields and the result will appear here automatically.
Calculate the nine-term Rothfusz regression and its difference from air temperature. This is the unadjusted regression, without low/high-humidity corrections or the full NWS algorithm’s preliminary branch. Categories use the index in °F and do not assess an individual person.
How it works
Formula and logic
T=1.8 t+32 °F; HI=−42.379+2.04901523 T+10.14333127 RH −0.22475541 T·RH −0.00683783 T²−0.05481717 RH²+0.00122874 T²RH+0.00085282 T·RH²−0.00000199 T²RH². Celsius=(HI −32)/1.8. Category thresholds are 80,90,103 and 125 °F.
Example
For 32 °C and 70% RH the regression gives≈40.409 °C or 104.736 °F: “danger”, since the index is≥103 °F. This does not mean 40 °C in completely dry air or a measured body temperature.
Fields and units
- Air temperature — °C
- Relative humidity — %
How to use
- — Enter shaded air temperature and RH measured at the same time.
- — Read the difference from the thermometer; some combinations give a lower index.
- — Check whether another application uses all NWS adjustments: identical numbers are not guaranteed.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- NWS: regression, separate adjustments and limitations
- Limitation
- Form bounds: t 20–60 °C,RH 0–100%, branch T≥80 °F. They do not validate every extreme combination; NWS warns about extrapolation. Sun, wind, activity, clothing and individual risk are not entered. Use official heat warnings for decisions.
FAQ
Why does humidity change the sensation so much?
Because the body cools by evaporating sweat. In damp air the sweat evaporates poorly, heat loss drops, and the same temperature becomes far harder to bear.
Why does it refuse below 26.7 °C?
Form bounds: t 20–60 °C,RH 0–100%, branch T≥80 °F. They do not validate every extreme combination; NWS warns about extrapolation. Sun, wind, activity, clothing and individual risk are not entered. Use official heat warnings for decisions.
How does the heat index differ from wind chill?
They cover opposite conditions. The heat index describes heat with humidity; wind chill describes cold with wind.
Where should the temperature be measured?
Enter shaded air temperature and RH measured at the same time. Form bounds: t 20–60 °C,RH 0–100%, branch T≥80 °F. They do not validate every extreme combination; NWS warns about extrapolation. Sun, wind, activity, clothing and individual risk are not entered. Use official heat warnings for decisions.