Free browser-based calculator
RAID & ZFS Capacity Calculator
Compare usable capacity, storage efficiency, and basic disk-failure tolerance before buying drives.
Capacity is not backup
RAID and ZFS redundancy can keep a pool available after certain disk failures. They do not protect against deletion, ransomware, theft, fire, controller failure, or every multi-disk failure. Keep a separate backup.
This is a planning estimate for equal-size drives. Filesystem metadata, reserved space, slop space, snapshots, compression, sector format, and mixed drive sizes can change the real result.
TB versus TiB
Drive manufacturers use decimal terabytes. Operating systems may show binary tebibytes while labeling them as TB. One marketed 1 TB is approximately 0.91 TiB before filesystem overhead.
How each layout is estimated
| Layout | Usable-capacity model | Failure note |
|---|---|---|
| RAID 0 | All drives | No disk-failure tolerance |
| N-way mirror | One drive | Can lose all but one mirror member |
| RAID 5 / RAIDZ1 | Drive count minus one | One disk |
| RAID 6 / RAIDZ2 | Drive count minus two | Two disks |
| RAID 10 | Half the drives | One disk guaranteed; more only in different mirror pairs |
| RAIDZ3 | Drive count minus three | Three disks |
Why real ZFS capacity is lower
The result is a purchasing estimate, not a promise of free space. ZFS reserves space for metadata and pool operation, and performance degrades when a pool is filled too far. Snapshots, special vdevs, spare drives, sector-size padding, and mixed-capacity disks also change the usable result.
Plan backup separately
Redundancy improves availability during certain disk failures. A backup protects a second copy with an independent failure path. Budget capacity for both: the live pool, snapshots or versioning, and at least one separate copy that is not continuously writable from the server.