For day-to-day purposes, if you are used to Fahrenheit but not Celsius or vice versa, and all you want to do is get a rough sense of how warm or cold it is outside without having to do arithmetic involving fractions in your head, then remember that there are two temperatures in Celsius that are roughly the same in Fahrenheit but with their digits transposed: 16° C ~ 61° F, and 28° C ~ 82° F. You can then roughly interpolate/extrapolate by about 2° F for every 1° C.
Bob and Doug Mackenzie thought me to roughly convert C to F by taking the temperature in Celsius, doubling it and then adding 30. It gets you in the ballpark.
For day-to-day purposes, if you are used to Fahrenheit but not Celsius or vice versa, and all you want to do is get a rough sense of how warm or cold it is outside without having to do arithmetic involving fractions in your head, then remember that there are two temperatures in Celsius that are roughly the same in Fahrenheit but with their digits transposed: 16° C ~ 61° F, and 28° C ~ 82° F. You can then roughly interpolate/extrapolate by about 2° F for every 1° C.
Also freezing is 0 in Celsius, so 32f is 0c. That one always helps me. Not as useful for converting c to f.
Alternatively, 100C is boiling, which is 212F.
Bob and Doug Mackenzie thought me to roughly convert C to F by taking the temperature in Celsius, doubling it and then adding 30. It gets you in the ballpark.