The conversion
18°F = −7.78°C
°C = (°F − 32) × 5/9, so (18 − 32) × 5/9 = −7.78.
As an outdoor air temperature, 18 °F sits in the freezing 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 | Celsius |
|---|---|
| 13 °F | −10.56 °C |
| 14 °F | −10 °C |
| 15 °F | −9.44 °C |
| 16 °F | −8.89 °C |
| 17 °F | −8.33 °C |
| 18 °F | −7.78 °C |
| 19 °F | −7.22 °C |
| 20 °F | −6.67 °C |
| 21 °F | −6.11 °C |
| 22 °F | −5.56 °C |
| 23 °F | −5 °C |
The other direction
−7.78 °C converts back to 18 °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.