Is RAID 5 or RAID 6 better for a small office NAS?

RAID 5 gives more space and survives one drive failure. RAID 6 gives up a second drive of space to survive two failures. With large drives, rebuilds take many hours and the remaining drives work hard throughout, so RAID 6 or Synology SHR-2 is the safer choice for business data on four or more drives.

Why does my NAS show less space than the drives I bought?

Two reasons. Some space is used for protection (parity or mirror copies), and drive makers count a terabyte as 1,000,000,000,000 bytes while NAS software counts in binary units. A 4 TB drive shows as about 3.64 TiB before any RAID is applied.

What is Synology SHR?

Synology Hybrid RAID splits drives into layers so mixed drive sizes are used more fully than standard RAID. SHR-1 protects against one drive failing and SHR-2 against two. It also makes it easier to grow the storage later by swapping in bigger drives.

Can I mix different drive sizes?

Yes, but standard RAID treats every drive as if it were the smallest one, so the extra space is wasted. Synology SHR uses most of it. The calculator shows how much space each layout leaves unused.

No. RAID keeps the NAS running when a drive fails. It does not protect against deleted files, ransomware, theft, fire or the NAS itself failing. Keep a separate backup, ideally including a copy in the cloud.

Free tool

RAID calculator for NAS drives

You bought four 8 TB drives and the NAS says you have about 21 TB. Where did the rest go? Pick your drives and a RAID type below to see the real usable space, how much goes on protection, and how many drives can fail before you lose data.

1. Your drives

4 drives

Drive 1Drive 2Drive 3Drive 4

2. RAID type

SHR-1Synology. Survives one drive failure and uses mixed drive sizes well.SHR-2Synology. Survives two drive failures, mixed sizes allowed.RAID 1Mirror. Every drive holds the same copy.RAID 5One drive of parity. Survives one failure.RAID 6Two drives of parity. Survives two failures.RAID 10Mirrored pairs, striped. Fast, survives one failure per pair.RAID 0Striping only. Fast, but one failure loses everything.

Usable space

12 TB

Shown as about 10.91 TiB in your NAS

Used for protection

4 TB

Parity or mirror copies

Drives that can fail

Without losing data

Usable 12 TB
Protection 4 TB
Not usable 0 TB
Total raw 16 TB

The per-drive view for SHR is simplified. RAID protects against a drive failing, not against deletion, ransomware or fire, so keep a separate backup.

How each RAID type works

RAID 1 (mirror): Two or more drives hold identical copies. Simple and safe for a two-bay NAS, but you only get the space of one drive.
RAID 5: Data and parity are spread across three or more drives. You lose one drive of space and can survive one failure.
RAID 6: Like RAID 5 with a second set of parity. You lose two drives of space and can survive two failures at once.
RAID 10: Drives are mirrored in pairs, then striped together. Fast for databases and virtual machines, but half the raw space goes on mirrors.
Synology SHR-1 and SHR-2: Synology’s own layout. Works like RAID 5 or RAID 6 but makes good use of drives that are not all the same size.
RAID 0: Striping with no protection. Fast, but if any drive fails everything is gone. Not for business files.

RAID 5 vs RAID 6: which should you choose?

RAID 5 is tempting because you only give up one drive. The catch comes when a drive fails. The NAS has to read every remaining drive in full to rebuild, and with today’s large drives that can take a day or more. If a second drive gives up during that time, the whole volume is lost. RAID 6 and SHR-2 keep a second drive of protection, so a rebuild is not a cliff edge.

Our rule of thumb for business files: two drives, use a mirror. Four or more drives holding work you cannot recreate, use RAID 6 or SHR-2. RAID 5 or SHR-1 is fine for media or data that also lives somewhere else.

Why the NAS shows less than you bought

Drive makers sell in decimal terabytes. NAS and computer software count in binary units (TiB), which are about 9% larger. So a 24 TB volume shows as roughly 21.8 TiB before anything is wrong. The calculator shows both figures.

RAID is not a backup

RAID keeps the NAS running when a drive dies. It does nothing if someone deletes a folder, ransomware encrypts the share, or the NAS is stolen. Our guide on backing up a NAS to the cloud covers the next step, and our business backup service can set it up for you.

Planning shared storage for an office? Read how to set up a NAS with folders and permissions for each team.

RAID calculator FAQs

Is RAID 5 or RAID 6 better for a small office NAS?: RAID 5 gives more space and survives one drive failure. RAID 6 gives up a second drive of space to survive two failures. With large drives, rebuilds take many hours and the remaining drives work hard throughout, so RAID 6 or Synology SHR-2 is the safer choice for business data on four or more drives.
Why does my NAS show less space than the drives I bought?: Two reasons. Some space is used for protection (parity or mirror copies), and drive makers count a terabyte as 1,000,000,000,000 bytes while NAS software counts in binary units. A 4 TB drive shows as about 3.64 TiB before any RAID is applied.
What is Synology SHR?: Synology Hybrid RAID splits drives into layers so mixed drive sizes are used more fully than standard RAID. SHR-1 protects against one drive failing and SHR-2 against two. It also makes it easier to grow the storage later by swapping in bigger drives.
Can I mix different drive sizes?: Yes, but standard RAID treats every drive as if it were the smallest one, so the extra space is wasted. Synology SHR uses most of it. The calculator shows how much space each layout leaves unused.
Is RAID a backup?: No. RAID keeps the NAS running when a drive fails. It does not protect against deleted files, ransomware, theft, fire or the NAS itself failing. Keep a separate backup, ideally including a copy in the cloud.

Want someone to set up the NAS properly?

We choose the drives and RAID layout, set up shared folders and permissions for each team, and add a cloud backup, for offices across London, Berkshire and Surrey.

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