RAID 6 Capacity Calculator
Estimate usable storage, parity overhead, and efficiency for a RAID 6 array.
What Is RAID 6?
RAID 6 is a storage configuration that stripes data across multiple drives and stores two independent parity blocks. The practical result is simple: your array can survive up to two drive failures at the same time without losing data availability.
Because two drives’ worth of space is reserved for parity, RAID 6 has lower usable capacity than RAID 5, but significantly better resilience for large arrays and high-capacity disks.
RAID 6 Capacity Formula
The core capacity formula is:
Usable Capacity (before filesystem overhead) = (Active Drives − 2) × Drive Size
- Active Drives = Total Drives − Hot Spares
- The 2 represents dual parity in RAID 6
- Minimum active drives for RAID 6 is 4
If you reserve additional space for filesystem overhead, snapshots, or thin provisioning buffer, the calculator also gives:
Net Usable = Usable Capacity × (1 − Overhead%)
How to Use This RAID Calculator
1) Enter physical configuration
Add the total number of installed drives, then the per-drive capacity in TB or TiB.
2) Account for hot spares
If one or more drives are dedicated as automatic replacement spares, include them in the hot spare field. They improve operational resilience but do not add active data capacity.
3) Add overhead reserve
Most environments should reserve at least a small percentage for metadata, snapshots, or performance headroom. A common planning value is 5% to 15% depending on workload and platform.
4) Read the output
The calculator shows total raw capacity, active array raw, parity cost, usable capacity, post-overhead capacity, and efficiency percentages.
Worked Examples
Example A: 8 × 12 TB, no spares
- Active drives: 8
- Parity equivalent: 2 drives
- Usable before overhead: (8 − 2) × 12 = 72 TB
- If 5% reserve: 72 × 0.95 = 68.4 TB net
Example B: 12 × 18 TB with 1 hot spare
- Total drives installed: 12
- Active drives in array: 11
- Usable before overhead: (11 − 2) × 18 = 162 TB
- With 10% reserve: 145.8 TB net
RAID 6 vs RAID 5 vs RAID 10
- RAID 5: Better capacity efficiency but only one-drive fault tolerance.
- RAID 6: Slightly lower capacity, but survives two simultaneous failures.
- RAID 10: Strong performance and rebuild profile, but usually only 50% raw capacity usable.
For large SATA/NL-SAS arrays, RAID 6 is often preferred because rebuild windows can be long and second-failure risk is non-trivial during rebuild.
Performance and Rebuild Considerations
Write penalty
RAID 6 parity calculations add write overhead. Sequential workloads are typically less affected than small random writes, but controller quality and cache policy matter a lot.
Rebuild time
As disk sizes grow, rebuilds can take many hours or even days under load. Dual parity gives a safety margin while the array is in degraded mode.
URE and risk modeling
Unrecoverable read errors become more relevant with large-capacity drives. RAID 6 helps reduce data-loss probability compared with single-parity designs, but it is not a substitute for proper backups.
Best Practices for RAID 6 Planning
- Use identical drive sizes in each RAID group.
- Keep firmware and controller settings consistent.
- Consider at least one hot spare for larger pools.
- Reserve free space (often 10–20%) to avoid performance collapse near full utilization.
- Always pair RAID with tested, versioned backups.
Quick FAQ
How many drives are required for RAID 6?
Minimum is 4 active drives.
Can RAID 6 survive two failed drives?
Yes. That is the defining benefit of RAID 6 dual parity.
Does RAID 6 replace backups?
No. RAID protects availability, not against deletion, ransomware, silent corruption, or site-wide disaster.