NAS planning example

12x 20TB RAID 10 NAS Calculator | Usable TB

Estimate usable TB, parity overhead, and fault tolerance for 12x 20TB in RAID 10. 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

240.00 TB

Usable Capacity

108.00 TB

Fault Tolerance

1 drive per mirror pair*

Efficiency

50.0%

Excellent random I/O and rebuild behavior; capacity is typically 50% of raw. This scenario applies a 10% filesystem reserve.

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Alternative Mode Comparison

Mode	Usable	Tolerance	Efficiency
RAID 5	198.00 TB	1 drive	91.7%
RAID 6	180.00 TB	2 drives	83.3%
RAID 10	108.00 TB	1 drive per mirror pair*	50.0%
RAID-Z1	198.00 TB	1 drive	91.7%
RAID-Z2	180.00 TB	2 drives	83.3%

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Sequential long-tail navigation

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FAQ

Should I optimize this 12-drive plan for capacity 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.

How much usable storage does 12x 20TB RAID 10 provide?

This NAS planning scenario estimates 108.00 TB usable after a 10% reserve from 240.00 TB raw.

How many disk failures can RAID 10 tolerate in this setup?

This setup can tolerate 1 drive per mirror pair*. Real-world survivability depends on mirror placement, rebuild stress, and drive health.

Why include a 10% reserve when planning NAS capacity?

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