"Calc" is an advanced calculator and mathematical tool that runs as
part of the GNU Emacs environment.

To run calc under Emac(speak) you'll load the prepare file by entering 
M-x load-library calc-prepare

or you'll add the line into the .emacs file in your home directory 
(load-library "calc-prepare")

Calc is started by typing
M-x calc


Check out the comprehensive calc info help files by changing into 
/usr/share/emacs/site-lisp/calc and typing 'info -f calc'.
Or you can fire up info under Emads by typing C-u C-h i <path_to_info_file>


From the introduction to the manual:

  "Calc" is an advanced calculator and mathematical tool that runs as
  part of the GNU Emacs environment.  Very roughly based on the HP-28/48
  series of calculators, its many features include:

    * Choice of algebraic or RPN (stack-based) entry of calculations.

    * Arbitrary precision integers and floating-point numbers.

    * Arithmetic on rational numbers, complex numbers (rectangular and
      polar), error forms with standard deviations, open and closed
      intervals, vectors and matrices, dates and times, infinities,
      sets, quantities with units, and algebraic formulas.

    * Mathematical operations such as logarithms and trigonometric functions.

    * Programmer's features (bitwise operations, non-decimal numbers).

    * Financial functions such as future value and internal rate of return.

    * Number theoretical features such as prime factorization and
      arithmetic modulo M for any M.

    * Algebraic manipulation features, including symbolic calculus.

    * Moving data to and from regular editing buffers.

    * "Embedded mode" for manipulating Calc formulas and data directly
      inside any editing buffer.

    * Graphics using GNUPLOT, a versatile (and free) plotting program.

    * Easy programming using keyboard macros, algebraic formulas,
      algebraic rewrite rules, or extended Emacs Lisp.