NAS planning example

6x 18TB RAID-Z2 NAS Calculator | Usable TB

Estimate usable TB, parity overhead, and fault tolerance for 6x 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.

Decision aid RAID level selector Reference Capacity matrix

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

108.00 TB

Usable Capacity

64.80 TB

Fault Tolerance

2 drives

Efficiency

66.7%

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	81.00 TB	1 drive	83.3%
RAID 6	64.80 TB	2 drives	66.7%
RAID 10	48.60 TB	1 drive per mirror pair*	50.0%
RAID-Z1	81.00 TB	1 drive	83.3%
RAID-Z2	64.80 TB	2 drives	66.7%

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Sequential long-tail navigation

← Previous: 6x 18TB RAID-Z1 NAS Calculator | Usable TB Next: 6x 20TB RAID 1 NAS Calculator | Usable TB →

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.

Is RAID-Z2 still viable 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.

Should I optimize this 6-drive plan for available space 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.

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.