This tool is to help you with planning and sizing (or verifying) your TMG 2010 configuration, and gives a calculation on the hardware configuration, number of concurrent users and bandwidth requirements using hardware, concurrent users or bandwith as starting point.
For its calculations the tool uses parameters like number of concurrent users and features you want to enable on the TMG. Regarding features you can make your own selection or use presets, e.g. “Mail Protection” for using TMG as an anti-spam/anti-malware e-mail gateway. The calculator has support for load-balancing and virtualization.
You can download the Forefront TMG 2010 capacity planning tool here.