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ZFS RAIDZ capacity calculator

How much a ZFS RAIDZ pool holds after parity and the space ZFS keeps free, how many drives can fail, and what one more drive adds later.

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Worked example: six 12 TB drives in RAIDZ2, which comes to 43.53 TiB. Each drive is priced at $391.33, the cheapest new internal 12 TB drive at Newegg on Oct 13, 2026. Type over any field to see your own.

43.53 TiB usable at most: 6 × 12 TB in RAIDZ2 keeps 4 drives of data (48.00 TB, 43.66 TiB), less the 128 GiB ZFS holds back so the pool never fills. Any 2 drives can fail without losing data. Drive cost: $2,347.98, or $49.06 per usable TB.
Usable TiB: 43.531 · Held back by ZFS, TiB: 0.125 · Parity, TiB: 21.828
Usable TiB: 43.531 · Held back by ZFS, TiB: 0.125 · Parity, TiB: 21.828

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Formula and limits

A RAIDZ group of N drives of size S with P parity drives holds about (N − P) × S and keeps working with any P drives failed; it needs at least P + 1 drives, and OpenZFS recommends 3 to 9 (OpenZFS, RAIDZ). Larger drives count only as much as the smallest.

ZFS then keeps the last 1/32 of the pool free so it never fills completely: at most 128 GiB, so from about 4 TiB of data space upward the amount held back stops growing (OpenZFS module parameters; the 128 GiB cap is in OpenZFS's source).

The result is an upper bound. Small blocks use more space for parity and padding than (N − P) × S suggests: OpenZFS's example of three drives in RAIDZ1 stores 4K records at 50% efficiency, the same as a mirror, against 66% for 128K records. Compression, snapshots and a special or log device are not modelled.

A RAIDZ group can be widened one drive at a time by attaching a drive. Its fault tolerance stays the same, and blocks written before keep their old data-to-parity ratio, spread over more drives; only new writes use the wider ratio. RAIDZ is not a backup.

Cost per usable TB = drive count × entered price ÷ usable TB (decimal).

Drives for the pool: hard drives for a NAS, priced today. For hardware RAID or mdadm, use the RAID capacity calculator.

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