RAID Capacity Calculator

Frequently Asked Questions

How much usable space does each RAID level give?

RAID 0 = n × size (no redundancy); RAID 1 = size of one (mirror); RAID 5 = (n−1) × size; RAID 6 = (n−2) × size; RAID 10 = (n ÷ 2) × size. The tool computes usable space and overhead.

How many drives can fail safely?

RAID 0: none. RAID 1/10: at least one (10 survives one per mirror). RAID 5: one. RAID 6: two. Rebuild stress can cause a second failure, which is why RAID 6 is preferred for large drives.

Why is my array smaller than the drive labels?

Marketing TB are 10¹² bytes while the OS shows TiB (2⁴⁰), a ~9% reduction, plus filesystem overhead. The calculator notes the TB-vs-TiB difference.

Is RAID a backup?

No. RAID protects against drive failure, not deletion, ransomware, or controller failure. Always keep separate backups in addition to RAID.

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Estimates for informational purposes only.

Important Disclaimer: Estimates for informational purposes only.

This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

RAID combines several physical disks into one logical volume, trading raw capacity for speed, redundancy, or both. This tool applies the standard per-level capacity formulas to your disk count n and per-disk size s: RAID 0 = n × s (pure striping, no protection); RAID 1 = (n ÷ 2) × s (mirroring); RAID 5 = (n − 1) × s (one disk's worth of distributed parity); RAID 6 = (n − 2) × s (double parity); RAID 10 = (n ÷ 2) × s (mirrored then striped); and JBOD = n × s (concatenation, no redundancy). It also reports parity overhead, how many drives you can lose, a rough read/write throughput multiplier, and storage efficiency as a percentage.

Worked Example

Four 4 TB drives in RAID 6: usable = (4 − 2) × 4 = 8 TB, with 8 TB consumed by double parity - only 50% efficiency, but the array survives any two simultaneous failures. The same four drives in RAID 5 yield (4 − 1) × 4 = 12 TB (75% efficient) but tolerate only one failure. In RAID 10 you also get 8 TB, with much faster writes than RAID 6 because there is no parity calculation - the classic capacity-vs-performance-vs-safety trade laid bare.

TB vs TiB - The Capacity You Actually Get

Drive makers sell decimal terabytes (1 TB = 10¹² bytes); operating systems usually report binary tebibytes (1 TiB = 2⁴⁰ bytes). The conversion is roughly TiB ≈ TB × 0.909, so an "8 TB" usable array shows as about 7.27 TiB in the OS - before filesystem metadata, reserved blocks, and snapshots shave off a few more percent. We display both so the number you provision against matches what users will see, not the marketing figure on the box.

Common Mistakes

RAID is not a backup: it survives drive failure, not deletion, ransomware, controller faults, or fire. Keep separate, offsite backups.

Single-parity on huge disks: RAID 5 rebuild of 12 TB+ drives runs for days; a second failure during rebuild destroys everything - use RAID 6.

Mixing disk sizes: every member is treated as the size of the smallest drive; the extra space on larger ones is wasted.

Forgetting the write penalty: parity RAID (5/6) is slow on small random writes due to read-modify-write - RAID 10 is far better for databases.

Choosing a Level

Pick by workload: RAID 10 for databases and VMs needing fast writes and quick rebuilds; RAID 6 for bulk archival and large drives where rebuild windows are long; RAID 5 only with small disks and good backups; RAID 1 for two-drive boot/OS resilience; RAID 0 only for scratch data you can lose; and JBOD when you want individual disks without any pooling. Whatever you choose, monitor SMART health and test restores - the worst time to learn your backup is broken is during a rebuild.

Frequently Asked Questions

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