Assuming you're talking about temperature conversion from Celsius to Fahrenheit and vice-versa... To convert from Celsius to Fahrenheit - Multiply by 1.8 then add 32.To convert from Fahrenheit to...
If we are talking musically, it is a acronym for the notes "in between the lines" of the bar in the Treble cleff.
Notes F, A, C and E; F being on the first space, E being on the top space.