The long awaited Exchange 2013 Server Role Requirements Calculator v5.1 has finally been released. Like its predecessor, the Exchange 2010 Mailbox Server Role Requirements Calculator, it was announced by Ross Smith IV at the Microsoft Exchange Team Blog.
There are several new features introduced by this version. Like you probably noticed from the name, the tool is no longer a mailbox exclusive calculator, it now includes client access servers and transport sizing (as the transport role is now part of the mailbox role).
For full details and prescriptive guidance on how to use the tool, read the Exchange 2013 Server Role Requirements Calculator Readme