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How much space is left after redundancy?

Enter the number of disks, their size and the layout. The calculator shows usable capacity in TB and TiB and how many disks may fail.

Updated: 25 Oct 2026 · Nothing you type leaves your browser. We store nothing and use no tracking.

With mixed sizes, enter the smallest disk

Result

Usable (TB)
16.00
Usable (TiB)
14.55
Raw capacity (TB)
32.00
Usable share
50.0 %
Failures always survived
2

Approximation: with small blocks, RAID-Z loses extra space to padding and parity, plus metadata and reserves.

What this is about

Four 8 TB disks make 32 TB – on the label. Depending on the layout, your NAS leaves you 16 to 24 TB of that, and the operating system then shows even less. Neither is shrinkage. It's arithmetic.

The first cut is redundancy. A mirror (RAID 1, ZFS mirror) stores everything more than once. RAID 5 and RAID-Z1 give up one disk's worth of space for parity, RAID 6 and RAID-Z2 two, RAID-Z3 three. In return, the same number of disks may fail without losing data. With RAID 10 and several mirrors it depends on which disks fail: lose a whole mirror and you lose the whole pool.

The second cut isn't one. Drive makers count terabytes of 10¹² bytes, many operating systems count tebibytes of 2⁴⁰ bytes and still label them "TB". That turns 8 TB into about 7.28 TiB. The calculator shows both.

For RAID-Z the result is explicitly an approximation. Small blocks cost extra space through padding and parity, and the file system keeps metadata and reserves on top. So leave some headroom. And whichever layout you pick: RAID is not a backup.

How we calculate

  • RAID 1 and ZFS mirror: a mirror of N disks holds one disk's capacity and survives N − 1 failures. Several ZFS mirrors of w disks each add up: usable = number of mirrors × disk size.
  • RAID 5: (N − 1) × disk size, from 3 disks, 1 failure. RAID 6: (N − 2) × disk size, from 4 disks, 2 failures.
  • RAID 10: N ÷ 2 × disk size with an even number of at least 4 disks. One failure is always safe, at best one per mirrored pair.
  • RAID-Z1, Z2 and Z3: approximately (N − P) × disk size with P = 1, 2 or 3 parity disks. At least P + 1 disks; OpenZFS recommends 3 to 9 per group.
  • TiB = TB × 10¹² ÷ 2⁴⁰, which is TB × 0.9095.
  • All disks the same size. With mixed sizes, the smallest disk sets the figure.

Sources

The sources back the formulas and limits. The selection, presets and rules of thumb are our own work.

  1. OpenZFSzpoolconcepts(7) (retrieved 25 Oct 2026)
  2. OpenZFSRAIDZ (retrieved 25 Oct 2026)
  3. Synology Knowledge CenterChoose a RAID Type (retrieved 25 Oct 2026)
  4. IECPrefixes for binary multiples (retrieved 25 Oct 2026)