RAID Capacity Calculator
Usable capacity and fault tolerance for RAID 0, 1, 5, 6 and 10 — how much of your raw disk space you actually get to use.
How it works
RAID 0: n×S · RAID 1: S · RAID 5: (n−1)×S · RAID 6: (n−2)×S · RAID 10: (n÷2)×S
Redundancy always costs capacity: the more failure you can survive, the more raw space you spend on parity or mirrors. RAID 0 keeps everything but protects nothing; RAID 1 mirrors, so half the space is a copy. RAID 5 sacrifices one disk’s worth to parity and survives one failure, RAID 6 sacrifices two and survives two, and RAID 10 mirrors then stripes for speed at the price of half the total. RAID is uptime insurance, not a backup — it does not protect against deletion, ransomware or a controller fault.
Worked example
Four 4 TB disks is 16 TB raw. In RAID 5 you lose one disk to parity, so usable capacity is (4 − 1) × 4 = 12 TB — 75% efficient, surviving any one disk failure. The same four disks in RAID 6 give (4 − 2) × 4 = 8 TB but tolerate two failures.
Frequently asked questions
Is RAID a backup?
No. RAID protects against a disk dying, but not against an accidental delete, a virus, fire, theft or a filesystem error — all of which hit every disk at once. Keep a separate, ideally offsite, backup as well.
RAID 5 or RAID 6 for large disks?
With big drives, favor RAID 6. Rebuilding a failed disk reads every remaining disk in full, and a second failure during that long rebuild wipes a RAID 5 array — the double parity of RAID 6 covers exactly that window.
Why does my array show less than the label capacity?
Two reasons stack: RAID overhead (this calculator’s figure) and the decimal-vs-binary gap, where a "4 TB" disk formats to about 3.64 TiB. Together they explain why usable space trails the sticker.
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This calculator is for educational purposes. Double-check important results before acting on them.