NAS planning example

12x 18TB RAID-Z2 NAS Calculator | Usable TB

Estimate usable TB, parity overhead, and fault tolerance for 12x 18TB in RAID-Z2. Includes reserve planning for NAS and homelab arrays.

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Planning route

Answer the capacity question, then validate the purchase path

Use this worked example as a numeric starting point, then validate the RAID choice, capacity reserve, hardware plan, and backup path.

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Editorial method

What this calculator can—and cannot—decide

The capacity model makes drive count, drive size, RAID layout, and reserve visible. It is a planning aid: it does not predict exact performance, rebuild duration, hardware compatibility, or the probability of data loss for a specific system.

Last reviewed: July 10, 2026. Product links remain neutral category searches until a partner relationship and page-level disclosure are in place.

OpenZFS checksums and scrubs Background on checksum verification, scrubs, and repair in suitably redundant ZFS pools. Read primary source ↗ TrueNAS data protection Official overview of scrubs, S.M.A.R.T. tests, snapshots, replication, and backup tasks. Read primary source ↗ CISA ransomware guidance Why recoverable, regularly tested backups should be separate from array redundancy. Read primary source ↗

Capacity Snapshot

Raw Capacity

216.00 TB

Usable Capacity

162.00 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	178.20 TB	1 drive	91.7%
RAID 6	162.00 TB	2 drives	83.3%
RAID 10	97.20 TB	1 drive per mirror pair*	50.0%
RAID-Z1	178.20 TB	1 drive	91.7%
RAID-Z2	162.00 TB	2 drives	83.3%

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Sequential long-tail navigation

← Previous: 12x 18TB RAID-Z1 NAS Calculator | Usable TB Next: 12x 20TB RAID 1 NAS Calculator | Usable TB →

FAQ

How much real-world usable storage does 12x 18TB RAID-Z2 provide?

This NAS planning scenario estimates 162.00 TB usable after a 10% reserve from 216.00 TB raw.

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.

Should I optimize this 12-drive plan for storage headroom or resiliency first?

For long-lived NAS pools, resiliency first is usually safer. Capacity can be expanded later, while a risky parity choice can force migration sooner.

Is RAID-Z2 still practical with 18TB drives?

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