Filesystems, Disks and Memory

I trying to calculate drive space with RAID. I know the equation is:

but, when I use that my calculations don't come out when I compare to what I already know exists:
I have 3 - 18.2gb SCSI drives that contain two logical drives C & D.
Using the math, (3-1)/3 * 18.2 = 12.1gb usable space per drive.

C has: 9.76gb
D has: 24.1gb

Drive D works out just fine but the C drive should have 12.1gb usable. I am so confused.

I need to know how to get the right numbers because my boss wants to know how much gain would come from intalling two additional 18.2gb drives.

Is there something I am missing?? I have researched 'til my eyes hurt because until a week ago, I haven't had to deal with calculating hard drive upgrades and such since my first year of college. HELP!!

Which RAID level are you using? Normally, RAID abstracts the disk space to the underlying OS and file system. If you are using 18GB drives, and using 3 drives, I assume RAID 5, then you will

A) have to see if your controller or driver will allow expansion to the RAID group.
B) Will always have N -1 capacity.

The requirements for RAID 5 are 3 drives minimum, with the capacity of a single drive being used across each of the drives for parity. This works out to N -1 all the time. If you had 9GB drives or 146GB drives it would not matter. You will always have N -1.

For example, 3 drives using 9GB drives will yield approximately 18GB of usable space. If you had 7 drives in a RAID 5 array, you will have (N -1, or 6 in that case * capacity) ~54GB of space.

The drive letters are being created based upon logical layout. That was chosen either by you or the program when it was created.

Basically, you as the user of a RAID should not care about individual drive usage. Part of the point of RAID is that it abstracts this. Your system should see 1 device. How you allocate it is up to you as the administrator. With that 1 large abstracted device, you could have carved the logical devices into 200MB chunks. That would have nothing to do with the array. That, I believe, is your problem.

Don't focus on usable space per drive, as you are confusing "per drive" with per logical device.