Heat index calculator
Enter the air temperature and either the relative humidity or the dew point. The heat index shows how hot it feels in the shade, with the US National Weather Service's risk level for that reading.
Heat index
121°F
49°C · air 96°F at 65% humidity
Danger
Heat cramps or heat exhaustion likely, and heat stroke possible with prolonged exposure and/or physical activity.
Rothfusz regression.
- 10%90
- 20%93
- 30%96
- 40%101
- 50%108
- 60%116
- 70%126
- 80%137+
- 90%137+
- 100%137+
- Under 80°F
- Caution
- Extreme caution
- Danger
- Extreme danger
Weather safety: a calculation, not a heat warning. Heat index values assume shade and light wind; in full sun they can be up to 15°F (8°C) higher. Follow local heat advice and seek medical help for signs of heat illness.
How the NWS calculates it
The calculator follows the Weather Prediction Center's description step by step. First it applies a simple formula, HI = 0.5 × {T + 61.0 + [(T − 68.0) × 1.2] + (RH × 0.094)}, with T in °F and RH in percent. If that gives 80°F or more, it switches to the Rothfusz regression, a nine-term equation fitted to the data behind Steadman's heat index. Two corrections follow: below 13% humidity between 80 and 112°F a small amount is subtracted, and above 85% humidity between 80 and 87°F a small amount is added. Readings in °C are converted to °F first.
The NWS notes the regression is accurate to about ±1.3°F. Against the 135 numbers on the NWS heat index chart, this calculator matches 125 exactly; the rest differ by up to 2°F, all in very humid air where the high-humidity correction applies, just as in the NWS's own calculator.
Risk levels and dew point
The levels are the NWS classifications: Caution from 80°F, Extreme Caution from 90°F, Danger from 103°F and Extreme Danger at 125°F or higher. To work from dew point instead, the tool turns it into relative humidity with the vapour-pressure formula the NWS calculator uses.
Sources
- NOAA Weather Prediction Center. The Heat Index Equation.
- NOAA Weather Prediction Center. Heat Index Calculator (dew point method).
- National Weather Service. Heat index chart and safety.
- National Weather Service Amarillo. Heat Index (classifications and accuracy).
Questions
Why does the sun matter?
Heat index values were devised for shady, light-wind conditions. The NWS says full sunshine can raise them by up to 15°F (8°C).
Why is the heat index lower than the air temperature?
In dry air the NWS formula can come out a little below the thermometer reading. That is how the index works; it does not make the heat safe.
Is a high heat index dangerous for everyone?
The NWS describes each level by its likely effects with prolonged exposure or physical activity. Check your local heat advice, and get medical help for signs of heat illness.