Just moments ago, Microsoft unleashed the all-new (or should I say re-branded) Exchange 2013 Server Role Requirements Calculator to the world.
Along with it’s release, Microsoft also made clear that multi-role deployments are still the way forward:

“Like with Exchange 2010, the recommendation in Exchange 2013 is to deploy multi-role servers”

Re-branding doesn’t mean that it changed completely. At least not the interface.
So don’t worry, the tool itself still looks very familiar and works very much in the same way as previous builds of the tool did.
Most of the changes were made under the hood. The tool now also provides sizing information for the Client Access Server role and takes into account the changes introduced by Exchange 2013’s new architecture.

Alongside a series of other improvements, a lot of changes were made in the area of High Availability calculations, including:

You can now specify the Witness Server location, either primary, secondary, or tertiary datacenter.
– The calculator allows you to simulate WAN failures, so that you can see how the databases are distributed during the worst failure mode.
– The calculator allows you to name servers and define a database prefix which are then used in the deployment scripts.
– The distribution algorithm supports single datacenter HA deployments, Active/Passive deployments, and Active/Active deployments.
– The calculator includes a PowerShell script to automate DAG creation.

In the event you are deploying your high availability architecture with direct attached storage, you can now specify the maximum number of database volumes each server will support. For example, if you are deploying a server architecture that can support 24 disks, you can specify a maximum support of 20 database volumes (leaving 2 disks for system, 1 disk for Restore Volume, and 1 disks as a spare for AutoReseed).

To download the Calculator and have a look at the full article for the Exchange team, have a look here: http://blogs.technet.com/b/exchange/archive/2013/05/14/released-exchange-2013-server-role-requirements-calculator.aspx