Usable RAID capacity depends on the level: RAID 0 = N × S, RAID 1 = S, RAID 5 = (N − 1) × S, RAID 6 = (N − 2) × S, RAID 10 = N ÷ 2 × S, where N is the number of drives and S the size of the smallest drive. Four 4 TB drives in RAID 5 give 12 TB usable.
RAID levels compared
| Level | Min drives | Usable | Failures survived |
|---|---|---|---|
| RAID 0 | 1 | N × S | 0 |
| RAID 1 | 2 | S | N − 1 |
| RAID 5 | 3 | (N − 1) × S | 1 |
| RAID 6 | 4 | (N − 2) × S | 2 |
| RAID 10 | 4 (even) | N ÷ 2 × S | 1 per mirror pair |
Why the OS shows less
Drives are sold in decimal TB. 12 TB usable = 12,000,000,000,000 bytes ÷ 1,099,511,627,776 = 10.91 TiB, which most operating systems display as "10.9 TB". File-system metadata takes a little more. See GiB to GB.
Mixed drive sizes
Classic RAID treats every drive as the size of the smallest one, so use the smallest drive size in the calculator. RAID is not a backup: it protects against drive failure, not deletion or corruption.
Related: TB to GB, GiB to GB, what is a terabyte, byte to MB converter.
FAQ
How much usable space does RAID 5 give?
RAID 5 gives (N − 1) × S. Four 4 TB drives give 12 TB usable.
How much usable space does RAID 6 give?
RAID 6 gives (N − 2) × S. Six 8 TB drives give 32 TB usable.
What is the usable capacity of RAID 10?
Half the raw capacity: N ÷ 2 × S. Four 4 TB drives give 8 TB.
How many drives can fail in RAID 6?
Any two drives can fail without data loss.
Why does my RAID show less than the calculator?
The calculator uses decimal TB as drives are sold; operating systems often show binary TiB and reserve space for file-system metadata.