NAS planning example

5x 4TB RAID 1 NAS Calculator | Usable TB

Estimate usable TB, parity overhead, and fault tolerance for 5x 4TB in RAID 1. 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

Turn the result into a storage brief

The capacity model makes drive count, drive size, RAID layout, and reserve visible. Use the resulting brief to check exact performance, rebuild duration, hardware compatibility, and the recovery plan for the chosen 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

20.00 TB

Usable Capacity

3.60 TB

Fault Tolerance

4 drives*

Efficiency

20.0%

Strong redundancy but low capacity efficiency. Great for small, critical datasets. This scenario applies a 10% filesystem reserve.

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Alternative Mode Comparison

Mode	Usable	Tolerance	Efficiency
RAID 5	14.40 TB	1 drive	80.0%
RAID 6	10.80 TB	2 drives	60.0%
RAID 10	N/A	N/A	N/A
RAID-Z1	14.40 TB	1 drive	80.0%
RAID-Z2	10.80 TB	2 drives	60.0%

NAS cluster navigation

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NAS Hub Calculator and buying path Learning Center 8-step guide path RAID Comparison Mode-by-mode matrix RAID Selector Choose by priority Capacity Matrix Citation-ready reference NAS FAQ Planning answers

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Sequential long-tail navigation

← Previous: 4x 20TB RAID-Z2 NAS Calculator | Usable TB Next: 5x 4TB RAID 5 NAS Calculator | Usable TB →

FAQ

Is RAID 1 still practical with 4TB 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 capacity?

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

How much usable storage does 5x 4TB RAID 1 provide?

This NAS planning scenario estimates 3.60 TB usable after a 10% reserve from 20.00 TB raw.

How many disk failures can RAID 1 tolerate in this setup?

This setup can tolerate 4 drives*. Real-world survivability depends on mirror placement, rebuild stress, and drive health.