Volume of a molecule contained within an isosurface

 Hi everyone,
 I wonder is someone knows of a free program that calculates the volume
 of a molecule contained within a particular density envelope (e.g. the
 0.001 a.u. density envelope) from a GAUSSIAN94 wavefunction or cube ?
 ( The volume I want to calculate is the INTEGRATED volume within the
 envelope of choice AND NOT the volume computed from intersecting spheres
 NOR the "solvent accessible volume").
 Thank you very much, I will summarize the answers.
   Cherif F. Matta		    	  tel. (905) 525-9140 ext. 22502
   Chemistry Department                    fax  (905) 522-2509
   McMaster University
   Hamilton, Ontario, CANADA L8S 4M1.