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.

Do you remember the Fibonacci sequence? You can use it to convert miles to kilometers .

2 mi ~= 3km

5mi ~= 8km

8mi ~= 13km

13mi ~= 21km

And so on.