RAID 6 Capacity Calculator
Estimate usable storage, parity overhead, and fault tolerance for a RAID 6 array.
What this RAID 6 disk calculator shows
RAID 6 is designed for resilience: it can survive any two disk failures in the active array. This calculator helps you plan the practical side: how much space you really get, how much is consumed by parity, and how storage changes when you reserve hot spares or account for filesystem overhead.
- Raw capacity: total active disk space before parity.
- Usable capacity: storage available to data after RAID 6 parity.
- Usable after overhead: a closer real-world estimate after formatting and metadata.
- Efficiency: the percentage of raw capacity that remains usable.
How RAID 6 capacity is calculated
Core RAID 6 formula
RAID 6 reserves the equivalent of two disks for distributed parity. So if your active array has N disks of equal size S, the formula is:
Usable Capacity = (N - 2) × S
For example, 8 active disks at 12 TB each gives:
Raw = 8 × 12 = 96 TB
Usable = (8 - 2) × 12 = 72 TB
Hot spares and why they matter
A hot spare is not part of the active RAID set until failure occurs. It improves recovery speed, but it does not contribute to usable space during normal operation. If you have 10 physical disks and assign 1 as a hot spare, your active RAID 6 disk count is 9 for capacity math.
RAID 6 planning tips before you buy hardware
- Minimum disk count is 4: RAID 6 needs at least four active disks.
- Use same-size drives: mixed sizes are limited by the smallest drive in most implementations.
- Expect long rebuilds: large-capacity drives can take many hours or days to rebuild.
- Use ECC RAM and quality controllers: parity arrays benefit from stable hardware.
- Always keep backups: RAID protects availability, not against deletion, malware, or disasters.
RAID 6 vs RAID 5 vs RAID 10
RAID 6
Best when you want strong fault tolerance on large arrays and can accept the two-disk parity overhead.
RAID 5
Higher efficiency than RAID 6, but only one disk fault tolerance—riskier with modern large drives.
RAID 10
Excellent performance and simple rebuilds, but generally only 50% usable capacity due to mirroring.
Frequently asked questions
How many disk failures can RAID 6 survive?
Any two simultaneous disk failures in the active array.
Is RAID 6 good for NAS systems?
Yes—especially for medium to large NAS pools where uptime matters and drive capacities are high. It is common in home labs, SMB file servers, and archival arrays.
Why does my OS show less space than the drive label?
Drive vendors use decimal units (TB), while many operating systems report binary units (TiB). The calculator displays both perspectives so your estimate matches what you see later.
Final note
Use this RAID 6 disk calculator as a sizing and budgeting tool. For production decisions, also account for workload, IOPS requirements, controller limits, backup strategy, and recovery objectives (RPO/RTO).