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RAID capacity calculator

Compare raw disk capacity with nominal usable space for equal-size RAID0, a two-disk mirror, RAID5 or RAID6. Free, account-free arithmetic in your browser.

Use one size for every disk

Inputs stay in memory on this page. No infrastructure is contacted and nothing is saved. Downloading creates a local text file.

2–1024 whole disks. RAID0 needs at least 2, RAID5 at least 3, RAID6 at least 4. The mirror option requires exactly 2.

All disks must have the same capacity. Enter one number greater than 0 and at most 1,000,000, with a decimal point and up to 6 decimal places. Mixed sizes, lists and spares are not supported.

Worked example: 4 equal 8 TB disks with RAID5, before overhead.

Raw capacity

32 TB / 29.10383 TiB

Nominal usable capacity

24 TB / 21.827873 TiB

RAID5: 4 × 8 TB. Parity or mirror allocation: 8 TB / 7.275958 TiB. Nominal usable share: 75%.

These are capacity illustrations, not available free space or a resilience guarantee. Values are rounded to 6 decimal places; very small values can round to zero in TB/TiB.

Formulas and units

Let N be the number of equal-size disks and S the capacity of one disk. Raw capacity is N × S. Nominal usable capacity is:

  • RAID0: N × S. No capacity is allocated to parity or mirroring.
  • Two-disk mirror: S. One disk-equivalent stores the second copy.
  • RAID5: (N − 1) × S. One disk-equivalent is allocated to distributed parity.
  • RAID6: (N − 2) × S. Two disk-equivalents are allocated to distributed parity.

Decimal GB and TB use powers of 1000: 1 GB = 1,000,000,000 bytes and 1 TB = 1,000,000,000,000 bytes. Binary GiB and TiB use powers of 1024: 1 GiB = 1,073,741,824 bytes and 1 TiB = 1,099,511,627,776 bytes.

The same byte capacity has different numbers in TB and TiB. For example, an 8 TB disk is approximately 7.275958 TiB. Converting the unit alone does not consume space.

Four equal 8 TB disks

Nominal capacities before overhead
Layout Disks Raw Nominal usable
RAID0 4 32 TB 32 TB
RAID5 4 32 TB 24 TB
RAID6 4 32 TB 16 TB
Two-disk mirror 2 16 TB 8 TB

With RAID5, (4 − 1) × 8 TB = 24 TB, approximately 21.827873 TiB. With RAID6, (4 − 2) × 8 TB = 16 TB, approximately 14.551915 TiB. The mirror example uses 2 disks; this worksheet does not extrapolate a 4-disk mirror.

What this excludes

Filesystem and metadata overhead, formatting, reserved space, hot spares, mixed disk sizes, controller limits and implementation-specific behavior can reduce usable space. This worksheet does not estimate these or recommend a ZFS topology, storage pool or live array configuration.

RAID is not a backup. Capacity arithmetic does not establish data durability, recovery time or protection against deletion, corruption or an outage. Check your implementation’s documentation and maintain independently recoverable backups.

ObserveifyNow reads infrastructure directly from your device. Manage free, account-free connections to inspect supported services. Protected Proxmox VE, Proxmox Backup Server and Beszel reads require the native app; the browser can read published Uptime Kuma status pages. This worksheet does not read or create arrays.