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12x 8TB RAID-Z2 NAS Calculator | Usable TB

Estimate usable TB, parity overhead, and fault tolerance for 12x 8TB in RAID-Z2. Includes reserve planning for NAS and homelab arrays.

Capacity Snapshot

Raw Capacity

96.00 TB

Usable Capacity

72.00 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	79.20 TB	1 drive	91.7%
RAID 6	72.00 TB	2 drives	83.3%
RAID 10	43.20 TB	1 drive per mirror pair*	50.0%
RAID-Z1	79.20 TB	1 drive	91.7%
RAID-Z2	72.00 TB	2 drives	83.3%

NAS Cluster Guides

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

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Sequential Long-Tail Navigation

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FAQ

How many disk failures can RAID-Z2 tolerate in this setup?

This setup can tolerate 2 drives. Real-world survivability depends on mirror placement, rebuild stress, and drive health.

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.

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.

How much effective storage does 12x 8TB RAID-Z2 provide?

This NAS planning scenario estimates 72.00 TB usable after a 10% reserve from 96.00 TB raw.