Heat Index Calculator: Feels-Like Temperature

About the Heat Index Calculator

The Heat Index Calculator tells you how hot it really feels when humidity is added to the air temperature. Sweat cools you as it evaporates, and damp air slows that down, so a humid 90 °F day feels much hotter than a dry one. Type the air temperature in °F or °C and the relative humidity, and the calculator shows the heat index in whole degrees, as the US National Weather Service (NWS) does, with its NWS level: Caution, Extreme caution, Danger or Extreme danger. Two other modes work out the Wind Chill and the Dew Point.

The heat index is an estimate for shade and a light wind. In full sunshine it can feel up to 15 °F hotter. It is not safety advice: follow the heat warnings of your local weather service, drink water and take breaks in the shade.

How to use it

The NWS method

With the air temperature $T$ in °F and the relative humidity $RH$ in percent, the NWS first works out a simple estimate, $HI = 0.5\,(T + 61 + 1.2\,(T - 68) + 0.094\,RH)$. If the average of that and $T$ is 80 °F or more, it uses the Rothfusz regression instead:

$$\begin{aligned} HI = {} & -42.379 + 2.04901523\,T + 10.14333127\,RH - 0.22475541\,T\,RH \\ & - 0.00683783\,T^2 - 0.05481717\,RH^2 + 0.00122874\,T^2 RH \\ & + 0.00085282\,T\,RH^2 - 0.00000199\,T^2 RH^2 \end{aligned}$$

When the humidity is below 13% and $T$ is 80 to 112 °F, it subtracts $\frac{13 - RH}{4}\sqrt{\frac{17 - |T - 95|}{17}}$. When the humidity is above 85% and $T$ is 80 to 87 °F, it adds $\frac{RH - 85}{10} \cdot \frac{87 - T}{5}$.

Worked examples

• 90 °F at 70% humidity: 106 °F, Danger.
• 110 °F at 40%: 136 °F, Extreme danger.
• 100 °F at 10%: 94 °F, after the dry-air adjustment.
• 82 °F at 95%: 94 °F, after the humid-air adjustment.
• 32 °C at 60%: 37 °C.

The levels are Caution from 80 to 90 °F, Extreme caution from 91 to 103 °F, Danger from 104 to 124 °F and Extreme danger from 125 °F. To convert other temperatures, use the Temperature Converter.

Privacy

Nothing is uploaded; every calculation runs in your browser and never leaves your device. The readings you type stay on this page and are gone when you close or reload it.

Frequently asked questions