ZFS Calculator
Pick your disk count, size, and vdev layout (mirror or RAIDZ1/2/3) to see usable pool capacity, how much goes to parity, and how many disks can fail. Capacity is shown in both decimal TB and the binary TiB your OS reports. Runs in your browser.
- Usable (TB)
- 8.0
- Usable (TiB)
- 7.3
- Raw total (TB)
- 16.0
- Efficiency
- 50%
- Disks for data
- 2
- Disks for parity
- 2
- Drives can fail
- 2
- Total drives
- 4
Two parity disks. Survives two failures. The sane default for most NAS builds.
A few ZFS realities the raw numbers miss
Plan around the vdev, not the pool. A ZFS pool is built from vdevs, and a pool dies if any single vdev dies. This calculator models one vdev, which is how most homelab pools are laid out. If you stack multiple vdevs, add their usable capacity together and keep in mind each vdev needs its own redundancy.
Leave headroom. ZFS slows down and copy-on-write fragments badly once a pool gets very full. Plan to stay under roughly 80% used, so treat the usable figure as a ceiling rather than a target.
RAIDZ1 is risky on large drives. During a rebuild every remaining disk is stressed, and a second failure loses the vdev. With big modern disks, RAIDZ2 is the safer default.
Choosing the platform to run the pool on? TrueNAS vs Unraid weighs ZFS against a flexible array, and Proxmox vs TrueNAS covers running ZFS under a hypervisor.