The conversion

9°F = −12.78°C

°C = (°F − 32) × 5/9, so (9 − 32) × 5/9 = −12.78.

As an outdoor air temperature, 9 °F sits in the bitter band. The other scale is on the dial beside it in the app, so a reading like this needs no conversion at all.

Convert something else

68°F

The range either side

Neighbouring values, for when the number you actually wanted was a degree or two away.

Fahrenheit to Celsius either side of 9 °F.
FahrenheitCelsius
4 °F−15.56 °C
5 °F−15 °C
6 °F−14.44 °C
7 °F−13.89 °C
8 °F−13.33 °C
9 °F−12.78 °C
10 °F−12.22 °C
11 °F−11.67 °C
12 °F−11.11 °C
13 °F−10.56 °C
14 °F−10 °C

The other direction

−12.78 °C converts back to 9 °F, which is the same relationship read the other way. Going from Celsius to Fahrenheit, multiply by 9/5 first and then add 32. Adding first is the usual mistake.

The full converter handles any value, in either direction, including kelvin, and shows the formula as it works.