RAID Capacity Calculator
How much usable space a RAID array gives you, how many disks can fail before you lose data, and what proportion of your disks are spent on redundancy.
Each RAID level trades capacity for safety.
How the raid capacity calculator works
Each RAID level trades capacity for safety. RAID 0 stripes with no redundancy — all capacity, no protection, and any failure loses everything. RAID 1 mirrors, halving capacity. RAID 5 uses one disk worth of parity, RAID 6 uses two, and RAID 10 mirrors striped pairs. Capacity is calculated from the smallest disk, because that is all a stripe can use from each member.
Formula: RAID 5: (n − 1) × size; RAID 6: (n − 2) × size; RAID 1/10: n × size ÷ 2
Worked examples
| Inputs | Usable capacity | Note |
|---|---|---|
| 4 × 4 TB in RAID 5 | 12 TB | 12 TB usable, one disk of parity |
| 4 × 4 TB in RAID 6 | 8 TB | 8 TB usable, two can fail |
| 6 × 8 TB in RAID 10 | 24 TB | 24 TB usable |
FAQFrequently asked questions
Is RAID a backup?
No. It protects against a disk failing, not against deleting a file, ransomware, corruption, theft or fire. Keep separate backups.
Which RAID level should I use?
RAID 6 or RAID 10 for anything important. RAID 5 leaves you exposed during the long rebuild after a failure, when a second disk is most likely to go.
Why is my array smaller than the disks add up to?
Parity or mirroring consumes capacity, and vendors sell decimal terabytes while the operating system shows binary tebibytes. Both figures are given above.
What if the disks are different sizes?
Every disk is used only up to the size of the smallest, so mixing sizes wastes the difference.
What happens during a rebuild?
The array reads every remaining disk in full, which is exactly when a second failure is most likely — the reason RAID 6 is preferred for large disks.
Where these figures come from
- IEC 80000-13 — Information science and technology (quantities and units) — the decimal (kB, MB) versus binary (KiB, MiB) prefixes used throughout
- RFC 4632 — Classless Inter-domain Routing (CIDR) — the address-plan arithmetic behind the subnet calculator
- RFC 1918 — Address Allocation for Private Internets — the private ranges the subnet calculator recognises
- NIST SP 800-63B — Digital Identity Guidelines, Authentication — length over composition rules; the basis of the password guidance here
- NIST SP 800-57 Part 1 — Recommendation for Key Management — key-strength comparisons used by the key-space calculator
- National Cyber Security Centre — password guidance — UK national guidance on password policy
Last checked: September 2026. Units follow the SI decimal convention (IEC 80000-13 defines the binary alternatives); network and security figures cite the defining standard.