RAID calculator.

Worked example

Four 8 TB drives (32 TB raw): RAID 0 gives 32 TB, RAID 5 gives 24 TB (21.83 TiB), RAID 6 gives 16 TB, RAID 10 gives 16 TB and RAID 1 gives 8 TB. With mixed drives of 4, 8, 8 and 12 TB (32 TB raw), RAID 5 treats every drive as 4 TB and keeps only 12 TB, while the SHR-style layout keeps 32 - 12 = 20 TB.

How it works

Classic RAID levels use the smallest drive as the size of every member. With n drives and smallest drive s: RAID 0 = n x s, RAID 1 = s (every drive holds a copy), RAID 5 = (n - 1) x s (one drive's worth of parity), RAID 6 = (n - 2) x s (two drives' worth of parity), RAID 10 = n / 2 x s (a stripe of mirrors). JBOD is simply the sum of the drives.

The SHR-style option is our approximation of a mixed-size layout with one-drive protection: usable = total of all drives minus the largest drive. It is not Synology's own calculation; for a Synology NAS use this result as a first pass and then check Synology's own RAID calculator. TiB is the same space counted in units of 2^40 bytes, which is why a "24 TB" result shows as 21.83 TiB.

Next steps: see how long a rebuild takes with the RAID rebuild time calculator, and how risky it is with the RAID rebuild URE risk calculator.

Estimates only. RAID is not a backup: it keeps an array running through a drive failure but does not protect against deletion, malware, fire or theft. Real arrays and drives vary, so use this for planning and check your own drive datasheets.

Sources and basis (opened 2026-10-08):

• Standard RAID levels (Wikipedia): RAID 0 and RAID 1 are limited by the smallest member disk (its example: 120 GB with 320 GB gives 240 GB); RAID 5 needs at least three disks and RAID 6 at least four.
• Nested RAID levels (Wikipedia): RAID 10 is a stripe of mirrors and needs a minimum of four drives.
• Non-RAID drive architectures (Wikipedia): JBOD means multiple hard drives exposed as individual devices.
• RAID Calculator (Synology): Synology recommends SHR or SHR-2 when you build a RAID from drives of different capacities to minimize wasted space. The exact SHR math here is our approximation.
• RAID (Wikipedia): an array is still exposed to fire, natural disaster, human error and controller failure, so backups should be kept, ideally off site.

FAQ

How much usable space do I get with RAID 5?

RAID 5 keeps (drives - 1) x the smallest drive. Four 8 TB drives give 24 TB usable out of 32 TB raw, which is 21.83 TiB.

What happens with different size drives in RAID?

Classic RAID levels treat every drive as the smallest one, so the extra space on bigger drives is wasted. For 4, 8, 8 and 12 TB in RAID 5 you keep 12 TB. A mixed-drive layout such as Synology Hybrid RAID is designed to waste less.

Why does my NAS show less than the usable TB here?

Drive makers count 1 TB as 10^12 bytes while many tools count in TiB (2^40 bytes), so 24 TB shows as 21.83 TiB. Filesystem overhead and system partitions take a little more.

Which RAID level should I pick?

It depends on how many drives you can afford to lose and how much space you need. RAID 5 loses one drive of space and survives one failure, RAID 6 loses two and survives two. Whatever you choose, keep a separate backup.

Is RAID a backup?

No. RAID keeps an array running through a drive failure but does not protect against deletion, malware, fire or theft. Keep separate backup copies.