RAID Capacity Calculator
Redundancy always costs capacity — the question is how much. This calculator takes a disk count, disk size, and RAID level (0, 1, 5, 6, 10, 15, 50, or 60) and returns the usable capacity, the raw capacity, and exactly how many gigabytes redundancy takes away.
Results
Enter data and click Calculate.
Raw vs usable capacity
Raw capacity = disk count × disk size. Usable capacity = what you can store after mirrors or parity. The gap is the price of redundancy — not “lost” gigabytes, but protection against disk failure.
RAID levels and tradeoffs
| Level | Idea | Usable capacity | Tolerance |
|---|---|---|---|
| RAID 0 | Stripe — max speed/capacity, zero protection; only for easily rebuilt data | 100% (n×s) | None |
| RAID 1 | Mirror | ~50% | 1 disk in the pair |
| RAID 5 | Single parity | (n−1)×s | 1 disk |
| RAID 6 | Double parity | (n−2)×s | 2 disks |
| RAID 10 | Stripe + mirror | ~50% | 1 disk per mirror pair |
| RAID 50 | Striped RAID 5 groups — large arrays wanting more usable space than RAID 10 | ~2×(n/2−1)×s | 1 disk / group |
| RAID 60 | Striped RAID 6 groups — large arrays needing extra rebuild safety | ~2×(n/2−2)×s | 2 disks / group |
Redundancy, disk failure, and backups
- After one disk fails, usable capacity does not change — the array runs degraded until you replace the disk and rebuild.
- During a RAID 5 rebuild there is no spare left — a second failure means data loss. Prefer RAID 6/10 on large disks.
- RAID ≠ backup. It protects against disk failure, not ransomware, deleted files, or a server-room fire.
Capacity vs resilience
- Max capacity with 1-disk tolerance — RAID 5.
- More headroom (2 failures) — RAID 6 (cost: one more disk for parity).
- Performance + simpler rebuild — RAID 10 (cost: ~50% capacity).
Examples
- 4 × 1 TB: RAID 5 → usable 3 TB, raw 4 TB; RAID 10 → usable 2 TB, raw 4 TB.
- 8 × 2 TB: RAID 5 → usable 14 TB; RAID 6 → usable 12 TB (more resilience, less space).
- 6 × 4 TB, RAID 5 → usable 20 TB, raw 24 TB, 4 TB spent on parity.
FAQ — RAID capacity
- Why do I lose capacity with RAID?
- Because some disks (or space) go to mirrors or parity so data can be rebuilt after a failure. That is the intentional cost of resilience.
- Which RAID level is better for capacity vs redundancy?
- RAID 5 maximizes space with 1-disk tolerance; RAID 6 spends more space to survive 2 failures; RAID 10 keeps ~50% capacity with strong performance.
- Does RAID replace backups?
- No. RAID does not protect against deletion, ransomware, or a site disaster. Off-site backup is a separate layer.
- How does one disk failure affect usable capacity?
- It does not shrink usable capacity — the volume runs degraded. Risk rises until you replace the disk and finish the rebuild.
- RAID 5 or RAID 10 for 4×1 TB disks?
- RAID 5: 3 TB usable; RAID 10: 2 TB usable, usually better write performance and a simpler rebuild.
- Minimum disks for RAID 50 / 60?
- RAID 50: ≥6; RAID 60: ≥8 (this calculator assumes a typical 2-group layout).
- Must disks be the same size?
- This calculator assumes yes. Mixed sizes usually treat every disk as the smallest — you waste the extra.
- Hardware vs software RAID?
- Hardware uses a controller; software (mdadm, ZFS) uses host CPU but is more flexible and less vendor-locked.