How RAID 6 capacity is calculated
RAID 6 uses dual distributed parity, which means the equivalent capacity of two drives is reserved for parity. This is why RAID 6 can survive any two drive failures in the active array.
If all drives are the same size, the core capacity formula is straightforward:
Usable RAID 6 Capacity = (Number of Active Drives − 2) × Drive Size
If you have hot spares, subtract them from installed drives first to get active drives:
Why your displayed capacity may look smaller
Many users expect an exact match between vendor drive size and what the operating system reports. In practice, differences happen due to:
- Decimal vs binary units (TB vs TiB)
- Filesystem metadata overhead
- Snapshot, thin provisioning, or reserved space policies
- Controller/OS formatting behavior
This calculator gives a practical estimate by showing both decimal (TB) and binary (TiB) figures, plus an optional reserve percentage.
Quick example
8 drives × 12 TB, no hot spare
- Active drives: 8
- Parity overhead: 2 drives (24 TB equivalent)
- Usable before reserve: (8 − 2) × 12 = 72 TB
If you apply a 5% reserve for operational headroom, estimated usable capacity is:
Planning guidelines for RAID 6
1) Keep drive sizes identical when possible
Mixed drive sizes usually force the array to operate at the smallest-drive size, reducing total usable capacity.
2) Account for rebuild risk windows
RAID 6 protects against two failures, but rebuilds on very large disks can still take significant time. During rebuild, performance is lower and risk exposure is higher.
3) Maintain free space
For many workloads, running arrays nearly full can hurt performance and recovery behavior. Keeping 10% to 20% free can be a healthy operational target depending on your platform.
4) Capacity is not backup
RAID improves availability, not backup. It does not protect against accidental deletion, ransomware, corruption propagation, or site loss.
Common mistakes this calculator helps avoid
- Forgetting that RAID 6 always consumes the equivalent of two drives for parity.
- Counting hot spares as active data capacity.
- Ignoring filesystem and operational reserve when estimating “real” usable storage.
- Confusing TB (decimal) with TiB (binary) in OS reporting.
RAID 6 capacity FAQ
What is the minimum number of drives for RAID 6?
Four active drives minimum. With 4 drives, 2 drives worth are parity and 2 drives worth are usable data.
How many drive failures can RAID 6 survive?
Any two simultaneous drive failures in the active array.
Is RAID 6 better than RAID 5 for large disks?
In many modern deployments, yes. RAID 6 adds a second parity block, improving resilience during long rebuild periods common with high-capacity drives.
Does this calculator include performance effects?
No. This tool focuses on capacity. Workload performance depends on controller design, cache, stripe size, filesystem, and read/write patterns.