We are in the process of creating a new AD for our organization. It would be single forrest, single domain, multiple sites topology (alas, i have no control over this decision). I am trying to spec the DC hardware and i am wondering if there is any tool like adsizer for windows 2008 server? Or is there any other way to calculate the size of the AD on disk? The ad will have about 25000 users, 4500 exchange mailboxes and about 30000 computer accounts and about 250 OUs. There will be about 36 or so sites. There will be 4 main DCs and 35 RODC in different physical locations. To recap i am trying to figure out the diskspace needed to store the AD db.