Change value automatically with drop down list

[saintmoin]

Hi
I want to change values with drop down list. For example drop down list has two options one in centimeter and another in inch.
So if I have already entered values with one of the options (suppose 73 in inch) and when I want to see it in other option I just select it in drop list and the cell changes its value accordingly (185.42 in centimeter). Is that possible?