NAS planning example

2x 4TB RAID 1 NAS Calculator | Usable TB

Estimate usable TB, parity overhead, and fault tolerance for 2x 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

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

8.00 TB

Usable Capacity

3.60 TB

Fault Tolerance

1 drives*

Efficiency

50.0%

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	N/A	N/A	N/A
RAID 6	N/A	N/A	N/A
RAID 10	N/A	N/A	N/A
RAID-Z1	N/A	N/A	N/A
RAID-Z2	N/A	N/A	N/A

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FAQ

How much real-world usable storage does 2x 4TB RAID 1 provide?

This NAS planning scenario estimates 3.60 TB usable after a 10% reserve from 8.00 TB raw.

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.

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 2-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.