Unraid Calculator: usable capacity with mixed drive sizes
By Devin Chua · Data checked by Owen Nakamura · Updated In Unraid, usable capacity is simply the sum of your data drives. A 16 TB, a 12 TB and two 8 TB drives with one parity drive give 28 TB usable, because the largest drive (16 TB) becomes parity and the rest keep their full size. Unlike RAID 5/6 or RAID-Z, Unraid does not size to the smallest drive, so mixed drive sizes cost nothing. Set your drives below to see usable capacity, fault tolerance and the cost to fill in Canadian dollars.
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Unraid always assigns parity to the largest drives. A parity drive must be at least as large as the largest data drive.
28 TB in drive-maker terms, which is simply the sum of your data drives. Parity uses 16 TB.
Drives: from CA$2,373 for 4 drives (CA$53.93/TB overall)
Mixed drive sizes cost nothing here. That is the appeal of Unraid: unlike RAID 5, RAID 6 or RAID-Z, Unraid does not size to the smallest drive. Every data drive contributes its full capacity, and you can add a single larger drive at any time.
Why Unraid is the exception
Every other calculator on this site levels down to the smallest drive in the array, because RAID and RAID-Z stripe data across all members. Unraid does not stripe. Each data drive holds whole files on its own filesystem, and one or two dedicated parity drives protect the set. That single design choice is why a pile of mismatched drives — the exact drives that waste capacity in RAID 5 — costs nothing here.
How Unraid capacity is computed
- Usable capacity: the sum of the data drives, full stop. Mixed sizes are fine.
- Parity: Unraid assigns the largest drive(s) to parity. A parity drive must be at least as large as the largest data drive.
- Fault tolerance: one parity drive survives one failure, two survive two.
The trade-off against RAID and ZFS
Flexibility is not free. Because Unraid reads a file from a single drive rather than striping across many, read and write speed is roughly one drive's worth, not the array's — fine for a media server on a gigabit network, less so for demanding workloads. If you want striped performance and are buying matched drives anyway, the RAID calculator (including SHR) or the RAID-Z calculator for ZFS is the better fit.
Grow one drive at a time
The everyday advantage of Unraid is upgrades. You can add a single larger drive whenever you need more space, without rebuilding the whole array — as long as no data drive exceeds the parity drive. When a data drive would be larger than parity, upgrade the parity drive first, then add the bigger data drive.
Unraid parity is still not a backup
Parity rebuilds a failed drive and nothing more. The one comfort is independence: lose more drives than your parity covers and you lose only those drives' files, not the whole array. That is better than a striped array's all-or-nothing failure, but it is not a backup. Keep a second copy elsewhere.
Frequently asked questions
How much usable space do I get in Unraid with mixed drives?
Simply the sum of your data drives. Unraid gives each data drive its own filesystem, so mixed sizes cost nothing: a 16 TB, a 12 TB and two 8 TB drives with one parity drive give 12 + 8 + 8 = 28 TB usable, because the largest drive (16 TB) becomes parity.
How big does the parity drive have to be?
At least as large as your largest data drive. Unraid always assigns the largest drives to parity, so if you add a data drive bigger than your current parity drive, you must upgrade parity first. That is the one hard rule in Unraid sizing.
Why does Unraid let me mix drive sizes when RAID does not?
Because Unraid does not stripe data across drives. Each data drive holds whole files on its own filesystem, and parity is computed across them. Nothing is sized to the smallest drive, so there is no wasted surplus — unlike RAID 5/6 or RAID-Z, which level down to the smallest member.
One parity drive or two?
One parity drive survives one failure; two survive two. With large drives the rebuild is slow and read-heavy, so a second parity drive is worth it once you have many drives or irreplaceable data. Two parity drives cost two whole drives of capacity, not a percentage.
Is Unraid parity a backup?
No. Parity rebuilds a failed drive; it does nothing against deletion, ransomware or theft. The one silver lining is that because each drive is independent, losing more drives than your parity covers only loses those drives' contents, not the whole array. You still need a real backup.
Devin Chua works out which drives, RAM and NVMe cache fit which NAS model at nasdrives.ca, and what the RAID choice means for usable capacity, checked against what is in stock on Amazon.ca.
Data checked by Owen Nakamura, Technical editor (data checking)