How much disk space do I need on my Exchange Server system?

A. This question is a hard one to answer—sort of like "how long is a piece of string?" The amount of disk space you need on a particular Exchange Server 5.5 or Exchange 2000 Server system depends on several factors:

If you know how many users you want the server to host and how much disk space each mailbox will be allowed, you can calculate the amount of storage you need as follows. For example, let's say you're building a server for 250 users, each with a maximum mailbox size of 50MB. 250 x 50 = 12,500, so you need roughly 12.5GB just for the private Information Store (IS). You must also factor in room for expansion (I always recommend adding at least 33 percent more disk for expansion), and you have to keep in mind that the Isinteg and Eseutil repair utilities might require up to twice the store size to do their magic. Given those requirements, 250 users at 50MB each might lead you to build a 36GB store, which I think would be reasonable.