RAID 6 Storage Calculator
Calculate usable storage in RAID 6 with dual parity protection. Survive up to 2 simultaneous drive failures.
✓ (N-2) × drive size formula
✓ Dual parity protection
✓ RAID 5 vs RAID 6 comparison
Distributes parity across all drives. Survives one drive failure. Good balance of capacity and protection.
Usable Capacity
11.82 TB
75.0% storage efficiency
RAID level comparison
| Level | Min | Fault tolerance | Capacity formula |
|---|---|---|---|
| RAID 0 | 2 | None | N × Size |
| RAID 1 | 2 | N−1 drives | 1 × Size |
| RAID 5 | 3 | 1 drive | (N−1) × Size |
| RAID 6 | 4 | 2 drives | (N−2) × Size |
| RAID 10 | 4 | 1/mirror pair | (N÷2) × Size |
| RAID 50 | 6 | 1/sub-array | (N−2) × Size* |
| RAID 60 | 8 | 2/sub-array | (N−4) × Size* |
| JBOD | 1 | None | N × Size |
| SHR | 1 | 1 drive (SHR-1) | ≈(N−1) × Size |
* Assumes 2 sub-arrays. N = number of drives.
RAID 6: Dual Parity Storage
RAID 6 extends RAID 5 by adding a second parity block, allowing the array to survive two simultaneous drive failures. This makes it the recommended choice for large drives and enterprise storage.
Capacity Formula
Usable = (N - 2) × Drive Size
4 × 4TB: (4-2) × 4 = 8TB usable (50%)
6 × 4TB: (6-2) × 4 = 16TB usable (67%)
8 × 4TB: (8-2) × 4 = 24TB usable (75%)
12 × 4TB: (12-2) × 4 = 40TB usable (83%)
RAID 5 vs RAID 6 Comparison
6 × 8TB drives:
RAID 5: 40TB usable, 1 drive fault tolerance
RAID 6: 32TB usable, 2 drive fault tolerance
Capacity difference: 8TB (one drive), but RAID 6 is dramatically safer during rebuilds
When RAID 6 Is Essential
- Drives larger than 4TB: Rebuild times exceed 24 hours, making a second failure likely
- Arrays with 6+ drives: More drives = higher probability of multi-drive failure
- Business-critical data: Where downtime or data loss has financial consequences
- Unattended systems: NAS boxes or remote servers where a failed drive may go unnoticed
📊 The Math on Rebuild Risk
With 8TB drives, a RAID 5 rebuild reads every sector of every remaining drive (~24–48 hours). The probability of encountering an unrecoverable read error (URE) during this process is significant. RAID 6 survives this scenario; RAID 5 does not.