RAID Usable Capacity: RAID 1, 5, 6, and 10, and TB vs. TiB
October 29, 2026
RAID combines multiple drives for speed, redundancy, or both. The usable capacity depends on the level and how much space is used for redundancy. With n drives of size S: RAID 0 (striping, no redundancy) gives n × S; RAID 1 (mirroring) gives S for a pair, regardless of the count; RAID 5 gives (n − 1) × S, with one drive's worth of parity; RAID 6 gives (n − 2) × S, with two; RAID 10 gives n ÷ 2 × S.
For four 4 TB drives: RAID 0 = 16 TB, RAID 5 = 12 TB, RAID 6 = 8 TB, RAID 10 = 8 TB.
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RAID 5 survives one drive failure, RAID 6 survives any two, RAID 10 survives one failure per mirrored pair (at least one, up to two if they are in different pairs), RAID 1 survives all but one drive, and RAID 0 survives none. During a rebuild after a failure, the remaining drives are stressed; with large drives, a rebuild can take many hours or days, and another failure or read error at that time can destroy a RAID 5 array. That is one reason RAID 6 or RAID 10 are preferred for large drives. RAID is not a backup: it doesn't protect against deletion, corruption, malware, or fire.
| RAID level | Usable formula | Four 4 TB drives | Failures survived |
|---|---|---|---|
| RAID 0 | n x S | 16 TB | 0 |
| RAID 1 (2-way mirror) | S | 4 TB (two drives) | 1 per pair |
| RAID 5 | (n - 1) x S | 12 TB | 1 |
| RAID 6 | (n - 2) x S | 8 TB | 2 |
| RAID 10 | n / 2 x S | 8 TB | 1 to 2 |
TB vs. TiB
Drive makers use decimal units: 1 TB = 10¹² bytes. Operating systems often report binary units: 1 TiB = 2⁴⁰ = 1,099,511,627,776 bytes. A 4 TB drive therefore appears as 4 × 10¹² ÷ 1.0995 × 10¹² = 3.64 TiB. The 12 TB RAID 5 array shows as 10.91 TiB. The difference is about 9% at this size. Filesystem overhead and reserved space reduce it further, so expect a few percent less than the computed figure.
Choosing a Level
RAID 1 or 10 for performance and simplicity with small arrays; RAID 5 for read-heavy storage with modest sizes; RAID 6 for large arrays of large drives. Hot spares speed up recovery. Mixed-size drives: classic RAID uses the smallest drive's capacity for all members, so 4 TB + 4 TB + 2 TB drives in RAID 5 yield (3 − 1) × 2 = 4 TB. Some systems (such as Synology Hybrid RAID or ZFS configurations) use space better.
Backups
Follow the 3-2-1 rule: three copies of data, on two different media, with one offsite. RAID improves availability but not backup. Monitor the drive health (SMART), and replace drives proactively as they age.
Frequently Asked Questions
How much space does RAID 5 provide?
(n − 1) times the size of one drive.
How many drives can RAID 6 lose?
Two.
Why does a 4 TB drive show as 3.64 TiB?
Because drive makers use decimal terabytes, while systems often show binary units.
Is RAID a backup?
No.
What happens with mixed drive sizes?
Classic RAID uses the smallest drive's size for each member.