How SHR uses mixed-size drives
Synology Hybrid RAID combines capacity from drives of different sizes into protected groups. The useful question is which parts of the larger drives have enough matching space on other drives to be protected.
Imagine two 4 TB drives and two 8 TB drives. All four contribute to the first 4 TB band. Only the two larger drives reach the next band. Before the calculator's system reservation, that inventory illustrates the difference between the modes:
| Mode | Lower band | Upper band | Theoretical data capacity |
|---|---|---|---|
| SHR-1 | Three 4 TB data equivalents, one for protection | One 4 TB data equivalent and one for protection | 16 TB |
| SHR-2 | Two 4 TB data equivalents, two for protection | Too few members for the model's protected upper band | 8 TB |
These are teaching numbers, not a claim about installed volume free space. The live result also deducts its modeled per-drive system reserve and shows the exact capacity bands.
Should I choose SHR-1 or SHR-2?
SHR-1 models tolerance of one drive failure in a protected layout. SHR-2 models two and requires at least four drives. More protection leaves less capacity for data; with mixed drives, it can also leave an upper band unused.
Choose the protection requirement before comparing the capacity. A larger usable result cannot tell you whether the recovery risk fits your needs. Neither mode replaces an independent backup, and this calculator does not estimate rebuild time or the chance of a second failure.
Why can DSM show a different capacity?
This is a fresh-pool model. It does not reconstruct the order in which drives were added or replaced in an existing pool. Expansion rules, the NAS model, and supported drive combinations still need checking in Synology's SHR guidance.
The modeled system reservation follows the approximate per-drive allowance described by Synology's RAID calculator. Filesystem metadata, snapshots, quotas, and other volume-level deductions are outside this estimate.
TB and TiB also describe the same bytes with different-sized units. Changing the display unit is not another capacity deduction. Enter the unit printed for each drive rather than treating a marketed TB number as TiB.
Check the intended pool before buying
Compare the inventory and SHR mode with your target NAS's supported configuration. For an existing pool, check the permitted expansion path in DSM's documentation before assuming a fresh-pool result is achievable by adding one disk.
For conventional RAID rather than SHR, use the RAID calculator. Calculation here stays in your browser. Share is an explicit action that places the mode and capacities in a URL; anyone with that link can read them.