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RAIDZ Calculator

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<p style="font:14px/1.6 system-ui,sans-serif;max-width:760px">Free <a href="https://dalil-tech.com/raidz-calculator">RAIDZ Calculator</a> by <a href="https://dalil-tech.com">Dalil Tech</a>.</p>

Compare all array levels

Set the level, drive count, and drive size to get usable capacity, how many drives can fail, and storage efficiency for a ZFS RAIDZ pool. The simple math is (N minus parity) times the smallest disk, but this also shows the real-world number, because ZFS gives you a few percent less than the clean formula.

Why the real number is lower than (N minus parity)

ZFS allocates in records and pads every stripe to stay aligned to the pool's ashift. With odd disk counts or small records, real usable space lands a few percent under the textbook estimate, and the wider the vdev the more it bites. So the calculator shows both: the clean formula you plan with, and the figure closer to what the pool actually reports once it is built. Size your purchase to the real number, not the marketing one.

I run SHR-1 on a Synology, not ZFS, so I will not pretend I daily-drive TrueNAS. The math here is the math; the padding estimate is why this calculator exists instead of a one-line formula.

Which RAIDZ level?

RAIDZ2 once your drives are 8 TB or larger, and it is the default here for that reason. A single-parity RAIDZ1 rebuild has to read every surviving drive end to end, and that full read is exactly when a second tired drive tends to drop. RAIDZ2 survives that. RAIDZ1 is fine on small drives or non-critical data you also back up; RAIDZ3 is for wide vdevs where the rebuild window stretches long. The RAIDZ1 vs RAIDZ2 vs RAIDZ3 guide makes the call, and the rebuild-risk page has the numbers. Whatever you pick, RAID is not a backup. Keep an offsite copy.

Common questions

How much usable space does RAIDZ2 give?

Roughly (N minus 2) times the smallest disk, so a six-bay pool of 12 TB drives is about 48 TB before ZFS overhead, and a few percent less after padding and slop. RAIDZ1 subtracts one disk, RAIDZ3 subtracts three.

Is RAIDZ usable space the same as RAID 5 or 6?

The capacity math is identical: RAIDZ1 matches RAID 5, RAIDZ2 matches RAID 6. What ZFS adds is per-block checksums and self-healing on a scrub, not more or less space.

How many drives can RAIDZ survive losing?

RAIDZ1 survives one drive failure, RAIDZ2 survives two, RAIDZ3 survives three. That is the number of parity disks in the vdev.