If you are not in Emacs at the moment but you wish to use Calc, you must start Emacs first. If all you want is to run Calc, you can give the commands:
emacs -f full-calc
emacs -f full-calc-keypad
which run a full-screen Calculator (as if by C-x * b C-x * c) or a full-screen X-based Calculator (as if by C-x * b C-x * k). In standalone operation, quitting the Calculator (by pressing q or clicking on the keypad <EXIT> button) quits Emacs itself.