Unraid Storage Calculator
Unraid Storage Calculator
Work out exactly how much usable storage an Unraid array gives you from any mix of drive sizes. Add your disks below, pick single or dual parity, and the calculator shows usable capacity, the parity overhead and how many drives you can lose. Because Unraid does not stripe data, it happily pools drives of different sizes, so you never waste the extra terabytes that traditional RAID would strand.
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How Unraid calculates usable capacity
Unraid keeps each file whole on a single drive and dedicates one or two drives entirely to parity. Usable space is therefore just the sum of your data drives, with the parity drive(s) removed. The catch is that each parity drive must be at least as large as your biggest data drive, so parity is always sized to the largest disk in the array.
Dual parity (2P): usable = sum of all drives − the two largest drives
Worked example
Say you have an 18 TB, a 12 TB and two 8 TB drives. With single parity the 18 TB drive becomes parity, leaving 12 + 8 + 8 = 28 TB usable and protection against one drive failure. Add a second parity drive (the 12 TB) and you drop to 8 + 8 = 16 TB usable, but now survive two simultaneous failures. Change the numbers in the widget above to model your own build.
Unraid vs traditional RAID for mixed drives
Traditional RAID 5 or RAID 6 treats every disk as if it were the smallest one, so a single 20 TB drive next to a stack of 8 TB drives wastes 12 TB. Unraid uses the full size of every data drive, which makes it the natural choice for home servers that grow one mismatched disk at a time. The trade-off is speed: Unraid reads and writes from a single drive at a time rather than striping across many, so raw throughput is lower than RAID 0, 10 or ZFS. For a media server or backup target that rarely matters; for databases or heavy virtualisation it might.
Want to compare Unraid against RAID 5, RAID 6, SHR or ZFS RAIDZ side by side? Use the full RAID calculator, read the Unraid parity explainer, or see how the operating systems stack up in Unraid vs TrueNAS vs OpenMediaVault.
Frequently asked questions
How does Unraid calculate usable capacity?
Usable capacity is the sum of all data drives, with the parity drive(s) excluded. Single parity makes the largest drive parity, so usable = total minus the largest drive. Dual parity removes the two largest drives.
Do Unraid drives need to be the same size?
No. Unraid pools drives of any size. The only rule is that each parity drive must be at least as large as the biggest data drive, so parity is always sized to the largest disk.
How many drives does Unraid need?
A minimum of one data drive plus one parity drive (two total) for single parity, or three drives for dual parity. A pool scales up to 28 storage devices.
Is Unraid the same as RAID?
No. Unraid stores each file whole on one drive and protects data with dedicated parity drives instead of striping. If more drives fail than you have parity, only the data on the failed drives is lost, not the whole array.
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