RAID 5 Capacity

When dealing with a RAID 5 storage system, how do you calculate the total storage capacity??

For example, if you have 5x 30GB HDDs one of these should be used for parity, so my guess would be the total capacity is 4x 30GB = 120GB. Is this correct? or am I missing something??

My boss seems to think there is a formula to calculate the actual capacity.

Yeah that is the std calculation fo raid 5 capacity

Capacity = n-1 x size

You are right in that one disk is used for striping/parity. It isn't physically one disk, it's just one disk worth of space, spread across the entire set of disks.