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12x 4TB RAID-Z2 NAS Calculator | Usable TB

Estimate usable TB, parity overhead, and fault tolerance for 12x 4TB in RAID-Z2. Includes reserve planning for NAS and homelab arrays.

Capacity Snapshot

Raw Capacity

48.00 TB

Usable Capacity

36.00 TB

Fault Tolerance

2 drives

Efficiency

83.3%

Popular TrueNAS default for medium arrays; dual-parity with good safety margin. This scenario applies a 10% filesystem reserve.

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Alternative Mode Comparison

Mode Usable Tolerance Efficiency
RAID 5 39.60 TB 1 drive 91.7%
RAID 6 36.00 TB 2 drives 83.3%
RAID 10 21.60 TB 1 drive per mirror pair* 50.0%
RAID-Z1 39.60 TB 1 drive 91.7%
RAID-Z2 36.00 TB 2 drives 83.3%

NAS Cluster Guides

NAS Hub RAID Level Compare RAID Capacity Matrix Planning Quick Sheet RAID Selector NAS FAQ

Related Long-Tail Calculators

2x 4TB RAID 1 2x 6TB RAID 1 2x 8TB RAID 1 2x 10TB RAID 1 2x 12TB RAID 1 2x 14TB RAID 1 2x 16TB RAID 1 2x 18TB RAID 1 2x 20TB RAID 1 3x 4TB RAID 1 3x 4TB RAID 5 3x 4TB RAID-Z1 3x 6TB RAID 1 3x 6TB RAID 5 3x 6TB RAID-Z1 3x 8TB RAID 1

Sequential Long-Tail Navigation

← Previous: 12x 4TB RAID-Z1 NAS Calculator | Usable TB Next: 12x 6TB RAID 1 NAS Calculator | Usable TB →

FAQ

Why include a 10% reserve when planning NAS storage headroom?

Keeping free space improves filesystem behavior for snapshots, metadata, and write performance. Full arrays often perform worse and rebuild more slowly.

How much real-world usable storage does 12x 4TB RAID-Z2 provide?

This NAS planning scenario estimates 36.00 TB usable after a 10% reserve from 48.00 TB raw.

Is RAID-Z2 still worth deploying with 4TB drives?

It can be practical, but larger drives increase rebuild windows. Validate parity choice and backup policy before committing to the final layout.

Can this calculator replace real-world benchmark and rebuild testing?

No. Use this page for pre-purchase sizing, then validate with workload benchmarks, SMART health policy, and a tested restore plan.