Hardware Requirements for exchange server 2013

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Component	Requirement
Processer	Minimum 1.6 GHZ or Faster It should be 64 bit
Memory	Minimum(Mailbox and Client Access combined )8 GB Ram Edge Transport   4GB minimum Recommended 8GB +10MB Per Mail box Means if you are using  100 mailboxes (100  x 10  = 1024 MB =1GB Now Total is 8 GB + 1GB
Available disk space	Minimum 50 GB free disk space on the drive where you install exchange server 200 MB Free disk space at the system drive (OS drive)
Drive	DVD-ROM Drive

Note: In exchange Server 2010 Ram is 4GB + 2mb per mail box

         In exchange server 2007 Ram is 2GB + 5mb per mail box