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Homelab · Tool · By Mohammed Almuhanna
unRAID, RAID, ZFS & SHR Storage Calculator
Compare all array levels
Pick an array level for same-size drives, or switch to Mixed drives to size a Synology SHR pool from disks of different sizes. You get usable capacity, how many drives can fail, storage efficiency, and cost per usable terabyte, in both marketed TB and the TiB your operating system actually reports.
What is Synology SHR, and should you use it?
SHR (Synology Hybrid RAID) is Synology's friendlier layer on top of standard RAID. Its one real trick is mixed drives: plain RAID 5 limits every disk to the size of the smallest one, so a 12 TB drive in an array of 4 TB drives only contributes 4 TB. SHR layers parity across the different drive sizes and reclaims most of that wasted space, while still surviving one drive failure. SHR-2 survives two.
Switch the calculator above to Mixed drives and pick SHR to watch it happen. A pool of 4, 4, 8, 8, 12, 12 TB gives 20 TB usable on RAID 5 but 36 TB on SHR. That gap is the problem SHR solves.
When should you use it? If your drive shelf is a pile of mismatched disks bought over years, SHR is the easy, space-efficient default, and it lets you grow by swapping in one bigger drive at a time. I run SHR-1 on my own Synology DS1522+. My drives are matched, so SHR and plain RAID 5 give the same usable space for me, but SHR keeps the door open to add a single bigger drive later without rebuilding the whole array. SHR is Synology-specific, though, so a standard RAID or ZFS layout is more portable if you ever migrate off Synology.
RAIDZ (ZFS): RAIDZ1 vs RAIDZ2 vs RAIDZ3
RAIDZ is ZFS's parity. RAIDZ1 keeps one parity disk so one drive can fail, RAIDZ2 keeps two, RAIDZ3 keeps three. The usable-space math is the same as RAID 5 and 6: take the drives, subtract the parity disks, multiply by the smallest disk. What ZFS adds on top is per-block checksums and scrubbing, so it catches silent corruption that plain RAID never sees. Switch the calculator to RAIDZ to get usable capacity and efficiency for your own disks.
I run SHR-1 on a Synology, not ZFS, so I will not pretend I daily-drive a TrueNAS box. On picking a level: RAIDZ2 is the safe default once your drives are 8 TB or larger, for the same reason single parity gets dangerous on big disks. A rebuild has to read every surviving drive in full, and that full read is exactly when a second tired drive tends to give out. The RAIDZ1 vs 2 vs 3 guide walks through the choice, and the rebuild-risk piece has the numbers.
unRAID storage: how capacity and parity work
unRAID does not stripe. Every disk holds whole files on its own filesystem, and one or two dedicated parity disks protect the array. Usable space is just the sum of the data disks; the parity disks add protection, not capacity. The one hard rule is that a parity disk has to be at least as large as your biggest data disk.
The trade is simple. You can mix any drive sizes and add one disk at a time, which is why people with a closet full of mismatched drives reach for it. You give up striping, so you read at the speed of one disk, not the whole array. Switch the calculator to unRAID for the usable total, and see RAID vs unRAID for which model fits how you actually use the box.
RAID calculator FAQ
How do I calculate usable RAID capacity?
For RAID 5, RAID 6, and ZFS RAIDZ, usable space is the number of drives minus the parity drives, times the size of the smallest drive. Mirrors (RAID 1 and RAID 10) give you half the raw capacity. unRAID is the sum of the data disks. The calculator above does all of them, including mixed-size SHR pools, and shows the result in both marketed TB and the TiB your operating system actually reports.
What does the SHR calculator do for mixed drives?
SHR reclaims the space plain RAID 5 wastes when your drives are different sizes. A pool of 4, 4, 8, 8, 12, 12 TB gives 20 TB usable on RAID 5 but 36 TB on SHR, and it still survives one drive failure (SHR-2 survives two). Switch to Mixed drives and pick SHR to watch the number change.
How is RAIDZ usable space calculated?
RAIDZ1 subtracts one disk for parity, RAIDZ2 subtracts two, RAIDZ3 subtracts three, then you multiply by the smallest disk in the group. ZFS adds checksums and scrubbing on top, which plain RAID does not have.
Is RAID a backup?
No. RAID is redundancy, not a backup. It keeps the array online when a drive dies; it does nothing for a fat-fingered delete, ransomware, or the whole box frying. I run SHR-1 and I am happy with single parity, but only because I keep a real offsite backup. Treat RAID as uptime, not safety.