Free tool · IT & Security
RAID & NAS capacity calculator
See how much usable space a NAS gives you and how many drive failures it can survive, for each RAID level.
How it’s calculated
The method, in plain terms.
- RAID 0 — striping, all capacity, no protection.
- RAID 1 — mirroring; capacity of one drive.
- RAID 5 — one drive’s worth of parity; survives one failure (3+ drives).
- RAID 6 — two drives of parity; survives two failures (4+ drives).
- RAID 10 — mirrored pairs striped together (even number, 4+ drives).
RAID keeps a NAS running when a drive fails. It is not a backup: deletion, ransomware and fire affect every drive. Keep a separate backup copy.
Which RAID level is best for a small office NAS?
RAID 5 is a common balance of space and protection for 3–5 drives; RAID 6 is safer for larger drives or more bays; RAID 1 suits two-bay NAS units.
Is RAID a backup?
No. RAID protects against drive failure, not accidental deletion, ransomware or theft. Use scheduled backups to another device or the cloud.
Why does my 4 TB drive show 3.6 TB?
Drive makers use decimal terabytes; operating systems use binary units (TiB). It is the same capacity measured differently.
Can I add drives to a RAID later?
Many NAS systems support expansion, but the method depends on the brand and RAID level. Plan capacity up front where possible.
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