The conversion

120°F = 48.89°C

°C = (°F − 32) × 5/9, so (120 − 32) × 5/9 = 48.89.

As an outdoor air temperature, 120 °F sits in the hot 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 120 °F.
FahrenheitCelsius
115 °F46.11 °C
116 °F46.67 °C
117 °F47.22 °C
118 °F47.78 °C
119 °F48.33 °C
120 °F48.89 °C
121 °F49.44 °C
122 °F50 °C
123 °F50.56 °C
124 °F51.11 °C
125 °F51.67 °C

The other direction

48.89 °C converts back to 120 °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.