Calculating automaticaly user input

Is there some way, and i know it is, just im not aware it of, to calculate automatically user input in second field.

So i want user to imput something in one field and as he types result is imidiately displayed in another field automaticaly.

First there woud be visible some default value, and calculated, and on click of a button, field got empty, and user can make his own input, and as he types, result is imidiately displyed in other field

thanks