The conversion

19°C = 66.2°F

°F = (°C × 9/5) + 32, so (19 × 9/5) + 32 = 66.2.

As an outdoor air temperature, 19 °C sits in the mild 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.

Celsius to Fahrenheit either side of 19 °C.
CelsiusFahrenheit
14 °C57.2 °F
15 °C59 °F
16 °C60.8 °F
17 °C62.6 °F
18 °C64.4 °F
19 °C66.2 °F
20 °C68 °F
21 °C69.8 °F
22 °C71.6 °F
23 °C73.4 °F
24 °C75.2 °F

The other direction

66.2 °F converts back to 19 °C, which is the same relationship read the other way. Going from Fahrenheit to Celsius, subtract 32 first and then multiply by 5/9 — doing it in the other order is the usual mistake and produces a plausible-looking wrong answer.

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