Unraid 6x 20TB RAID-Z2 NAS Calculator

Estimate usable TB, parity, and fault tolerance for Unraid NAS users using 6x 20TB in RAID-Z2.

Capacity Snapshot

Raw Capacity

120.00 TB

Usable Capacity

72.00 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	90.00 TB	1 drive	83.3%
RAID 6	72.00 TB	2 drives	66.7%
RAID 10	54.00 TB	1 drive per mirror pair*	50.0%
RAID-Z1	90.00 TB	1 drive	83.3%
RAID-Z2	72.00 TB	2 drives	66.7%

Unraid Planning Notes

Unraid users often optimize for incremental growth and flexible disk replacement, where parity planning and usable capacity targets evolve over time rather than all at once.

• Capacity plans should include phased drive upgrades, not only day-one layout.
• Use parity-protected growth steps to avoid risky bulk migrations.
• Track cache and parity pressure separately from archive data growth.

Brand / Region Glossary

Parity Disk

Dedicated parity disk(s) protecting data disks in the Unraid array model.

Cache Pool

Fast SSD/NVMe pool used for writes and application workloads.

Mover

Scheduled process that migrates data between cache and array tiers.

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

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FAQ

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.

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.

Is RAID-Z2 still worth deploying with 20TB drives?

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

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.