5 x 6Tb in Raid5 = 21.8 TB.. Why not 25Tb?

5 x  6Tb in  Raid5  =  21.8 TB…    Why not 25Tb ?

The 5 drives WD Red  6tb are new. 

I have no previous experience.

I installed them in an 5bay enclosure ( Mobius, by OyenDigital )

made a “clear Raid” procedure.

The whole thing seems to work just perfectly. 

But I dislike to have lost so much space before even beginning:

I invested 30 TB and expected 25 in an Raid5 configuration.

Instead, Win7 analyzes that there are only 21.8 available.

Is that normal ?

Thank You for any hints !

Welcome to the Community.

By default, Windows displays hard drive capacity using binary numbers as opposed to decimal numbers. A 6TB hard drive will be reported by Windows as 5.58TB.

RAID 5 usually reserves around 20%-25% of all accumulated storage for parity.

5.58 x 5 = 27.9

27.9 x RAID 5 reserved % = Actual usable space.

A 6TB hard drive would be reported by Windows as 5.46TiB.

RAID 5 reserves the capacity of one drive for parity purposes. The available storage space of a 5 x 6TB RAID 5 array corresponds to 4 x 6TB.

4 x 6TB = 24TB = 21.8TiB

The minumum number of drives in a RAID 5 configuration is 3, in which case parity storage consumes 33% of the total capacity.


“RAID 5 consists of block-level striping with distributed parity. … It requires that all drives but one be present to operate. Upon failure of a single drive, subsequent reads can be calculated from the distributed parity such that no data is lost. RAID 5 requires at least three disks.”

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