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Homelab · Tool · By Mohammed Almuhanna
unRAID Calculator
Compare all array levels
Add your drives, any sizes you like, and get usable capacity for unRAID with one or two parity disks. This opens in mixed-drive mode because that is what unRAID is for: every data disk keeps its full size, and one or two parity disks (which must be at least as large as your biggest data disk) protect the rest.
How unRAID space works, and where it wins
unRAID does not stripe. Each data disk holds its own files, and a dedicated parity disk (or two) rebuilds any single failed disk. Usable space is simply the sum of your data disks, and you give up one disk to parity, or two for dual parity. Because there is no striping, you can mix a 4 TB, an 8 TB, and a 14 TB drive and each contributes its whole size. The one rule is that a parity disk must be at least as big as your largest data disk.
The quiet advantage shows up when things go wrong past your parity. In striped RAID or RAIDZ, losing more drives than you have parity loses the whole array. In unRAID, because each disk is independent, you only lose the files on the disks that actually died, the rest are still readable. You pay for that with speed: reads and writes run at single-disk pace, not the combined speed striping gives you.
unRAID or RAID?
unRAID fits a media-and-files server built from a pile of mismatched drives you add to over time, one disk at a time, where single-disk speed is fine and easy expansion matters more than raw throughput. Striped RAID or ZFS fits when you want performance and are buying matched drives. I run a Synology on SHR-1 myself, which is a different middle path, and the RAID vs unRAID comparison lays out which model fits which person. Whichever you pick, it is redundancy, not a backup. Keep an offsite copy.
Common questions
How is unRAID usable space calculated?
It is the sum of your data disks. One disk goes to parity (or two for dual parity), and every remaining disk contributes its full size, whatever mix of sizes you use. There is no striping penalty capping disks to the smallest one.
Can unRAID mix different drive sizes?
Yes, that is its main strength. Each data disk keeps its full capacity, so a 4 TB, 8 TB, and 14 TB drive all count in full. The only requirement is that a parity disk is at least as large as your largest data disk.
What happens if more disks fail than unRAID has parity?
You lose only the files on the disks that actually failed, not the whole array, because unRAID does not stripe data across drives. Striped RAID and RAIDZ lose everything past their parity limit. That partial-loss behaviour is a real reason people pick unRAID.