RAID Capacity
Usable capacity, overhead, efficiency and fault tolerance of a RAID 0, 1, 5, 6 or 10 array from disk count and size — in decimal TB as sold and TiB as shown.
The usable space in TB and TiB, how much goes to redundancy, how many disks may fail, and every level compared at the same disks.
Example: 4 × 4 TB in RAID 5 gives 12 TB usable (10.91 TiB), 75 % efficient, one disk may fail; in RAID 6, 8 TB and two disks; in RAID 10, 8 TB.
Definitions,
not benchmarks.
What each RAID level spends on redundancy, why the smallest disk sets the array, the TB-versus-TiB gap that looks like missing space, and what capacity does not tell you.
Level by level
RAID 0 stripes every disk: n × size usable, no redundancy. RAID 1 mirrors: one disk’s worth usable however many copies, n − 1 may fail. RAID 5 keeps one disk of distributed parity: (n − 1) × size, one may fail. RAID 6 keeps two: (n − 2) × size, two may fail. RAID 10 mirrors pairs and stripes across them: half usable, one guaranteed, up to n ÷ 2 if each failure is in a different pair. Every member is counted at the smallest disk’s size, as arrays do.
TB against TiB
Drives are sold in decimal terabytes (10¹² bytes); operating systems report binary tebibytes (2⁴⁰ bytes) and often label them TB. A 4 TB drive is 3.64 TiB, so a 12 TB array shows as 10.91 TiB — nothing is missing. The page reports both so the number you see in the OS matches.
Spares and the rebuild
Hot spares sit outside the array and add nothing to capacity; they shorten the window before a rebuild starts. The warnings note where a level’s risk concentrates: RAID 0 has none, RAID 5 with large disks spends a long rebuild one failure from loss, and no RAID replaces a backup.
What the number does not settle
Performance, rebuild time, unrecoverable-read-error exposure, controller or filesystem overhead (ZFS, Btrfs and hardware controllers each reserve some), and the reserved space of the filesystem itself are outside this page. Inputs stay in your browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
Last reviewed 18 September 2026. How results are checked: How we verify.