QNAP RAID calculator: usable capacity under QTS
By Devin Chua · Data checked by Owen Nakamura · Updated With 4 × 8 TB in RAID 5, a QNAP NAS keeps about 21.83 TiB usable (24 TB in drive-maker terms), and a single drive failure is survivable. Worth knowing: QTS has no SHR. Mixed drive sizes cost you real capacity here, because every drive counts only as much as the smallest in the array. The calculator below shows only the RAID types QTS actually offers, priced to fill in Canadian dollars.
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RAID 1 (Mirroring)
That is 8 TB the way drive makers label capacity. Your NAS shows you the smaller number because it counts in powers of two.
Drives: from CA$2,016 for 4 × 8 TB (CA$63.01/TB overall)
What QTS does differently with RAID
QNAP has no SHR equivalent. Its 'flexible volumes' sound like one but do something else: they distribute space dynamically between volumes inside a pool, they do not rescue the surplus of mismatched drives. QNAP's own FAQ states that with mixed sizes every drive is counted down to the smallest.
Third-party drives in a QNAP NAS
QNAP locks no drives. The compatibility list is guidance; QNAP only warns that unlisted drives may affect stability. Install QuTS hero instead of QTS and you are on ZFS — then the RAID-Z calculator applies, not this one.
Calculate for your exact model
Bay count matters: a two-bay unit cannot do RAID 5, which needs at least three drives. So each QNAP model has its own calculator with the bays already set:
And then the drives
Which drives belong in a QNAP NAS is covered in the buying guides. The short answer: CMR, not SMR, rated for 24/7 use. Why that is not a detail but decides whether a rebuild succeeds is explained under CMR vs SMR.
Frequently asked questions
Which RAID types does QNAP support?
QTS offers Basic, JBOD, RAID 0, 1, 5, 6 and 10, depending on how many bays your model has. A two-bay unit cannot do RAID 5, which needs at least three drives.
Does QNAP have an equivalent to Synology's SHR?
No. QTS has no flexible RAID, so mixed drive sizes cost you real capacity — every drive counts only as much as the smallest in the array. If you have a stack of mismatched drives, Synology (SHR), TerraMaster (TRAID) or Unraid are the more economical platforms.
Can I put third-party drives in a QNAP NAS?
QNAP locks no drives. The compatibility list is guidance; QNAP only warns that unlisted drives may affect stability. Install QuTS hero instead of QTS and you are on ZFS — then the RAID-Z calculator applies, not this one.
How much storage is left with 4 × 8 TB in a QNAP NAS?
In RAID 5, about 21.83 TiB usable — 24 TB in drive-maker terms. One drive goes to parity and a single failure is survivable. The NAS shows less than the label because makers count in decimal and the NAS in powers of two.
Which drives belong in this NAS?
CMR NAS drives rated for 24/7 use: Seagate IronWolf, WD Red Plus, Toshiba N300 or their Pro variants. SMR drives do not belong in any RAID array, because they collapse dramatically during a rebuild after a failure.
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)