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SHR Calculator

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Compare all array levels

Add your actual drives, mismatched sizes and all, and get usable capacity for Synology SHR-1 and SHR-2. This opens in mixed-drive mode because that is SHR's whole point: it reclaims the space plain RAID 5 throws away when your disks are not the same size. It also shows the figure DSM will really report, which is a little under the raw math.

What SHR actually does for you

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 and the rest is wasted. SHR layers parity across the different sizes and reclaims most of that space while still surviving one drive failure (SHR-2 survives two). A pool of 4, 4, 8, 8, 12, 12 TB gives 20 TB usable on RAID 5 but about 36 TB on SHR. That gap is the entire reason SHR exists, and the calculator shows it live when you add mismatched drives.

Why DSM shows a bit less than the math

Synology carves a small system partition off every drive and keeps a metadata reserve, so the number Storage Manager reports lands under the clean parity math. The calculator shows both, the raw figure and the DSM-realistic one, so you are not surprised when the box reports less than a simple SHR formula promised. I run SHR-1 on my own Synology DS1522+, and matched drives mean SHR and plain RAID 5 give me the same space, but SHR keeps the door open to drop in one bigger drive later without rebuilding the array. One limit: SHR is Synology-only, so a standard RAID or ZFS layout is more portable if you ever leave Synology. And SHR is redundancy, not a backup. Keep an offsite copy.

Common questions

How is SHR usable space calculated?

SHR layers parity across drives of different sizes instead of capping everything to the smallest disk. Usable space is close to the sum of all drives minus the largest one (for SHR-1) or the two largest (for SHR-2), then a little less after Synology's system partition and metadata reserve.

Is SHR better than RAID 5?

For mixed drive sizes, yes, it reclaims space RAID 5 wastes and lets you grow one bigger drive at a time. For a set of identical drives it gives the same usable space as RAID 5, so the only gain there is easier future expansion. The tradeoff is that SHR is Synology-specific and not portable.

What is the difference between SHR-1 and SHR-2?

SHR-1 keeps one drive of parity and survives one failure; SHR-2 keeps two and survives two, at the cost of one more drive of capacity. SHR-2 is the safer choice once you have many drives or very large ones.