Mpexpr adds two new commands to Tcl, 'mpexpr' and 'mpformat'.  Mpexpr works 
much like Tcl's  native 'expr', but does all calculations using an arbitrary 
precision math package.  Mpexpr numbers can be any number of digits, with any 
decimal precision.  Final precision is controlled by a Tcl variable 
'mp_precision', which can be any reasonable integer, limiting only the
number of digits to the right of the decimal point. 

Mpformat works much like Tcl's 'format', except it formats multiple
precision numbers in a variety of formats.

Mpexpr also includes most math functions provided by 'expr', as well
as several new functions.  Mpexpr also supports Tcl variables and
nested evaluation, just like 'expr':

WWW: http://www.neosoft.com/tcl/ftparchive/sorted/math/mpexpr-1.0