Tools / RAID calculator
RAID calculator: usable capacity and fault tolerance
Work out usable capacity, efficiency and fault tolerance for RAID 0, 1, 5, 6, 10 and ZFS RAIDZ from the number and size of your drives. Free, no sign-up.
Your drives
This is an example. Change anything to make it yours.
If your drives are different sizes, enter the smallest. Classic RAID uses every drive as if it were that size.
What you get
- Usable capacity
- 24 TB
- As your OS shows it
- 21.83 TiB
- Raw capacity
- 32 TB
- Efficiency
- 75%
- Fault tolerance
- 1 drive can fail
One drive of parity. A good balance for a few drives, but with large drives a second failure during a long rebuild is a real risk.
Every level with 4 drives
| Level | Usable | Efficiency | Fault tolerance |
|---|---|---|---|
| RAID 0 (stripe) | 32 TB | 100% | None: one failure loses everything |
| RAID 1 (mirror) | 8 TB | 25% | 3 of 4 drives can fail |
| RAID 5 (single parity) | 24 TB | 75% | 1 drive can fail |
| RAID 6 (double parity) | 16 TB | 50% | 2 drives can fail |
| RAID 10 (striped mirrors) | 16 TB | 50% | 1 drive guaranteed, up to 2 if each is in a different pair |
| ZFS RAIDZ1 | 24 TB | 75% | 1 drive can fail |
| ZFS RAIDZ2 | 16 TB | 50% | 2 drives can fail |
| ZFS RAIDZ3 | Not possible | Needs at least 5 |
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How RAID capacity is worked out
Every layout treats all drives as the size of the smallest one, then spends some of the space on redundancy. RAID 5 spends one drive on parity, RAID 6 spends two, RAID 10 mirrors every drive so half the space is spare, and a RAID 1 mirror keeps the space of a single drive however many copies there are.
A worked example: four 8 TB drives in RAID 5 hold 3 × 8 = 24 TB. Your operating system will report about 21.8 TiB, because it counts in binary units. In RAID 6 the same drives hold 16 TB and survive two failures; in RAID 10 they also hold 16 TB, with faster rebuilds.
RAID keeps a system online when a drive fails. It is not a backup: deleted files, ransomware and a failed controller affect every drive at once.
Questions about this tool
How do I calculate RAID capacity?
Multiply the number of drives by the size of the smallest one, then take off the redundancy: nothing for RAID 0, one drive for RAID 5, two for RAID 6, and half for RAID 10. A mirror (RAID 1) gives the space of a single drive.
Why is my usable space lower than the calculator says?
Drives are sold in decimal terabytes (1 TB is 1,000,000,000,000 bytes) but most operating systems report binary tebibytes, so 1 TB shows as about 0.91 TiB. File systems and ZFS also reserve some space. The calculator shows both units.
Is RAID a backup?
No. RAID keeps a system running when a drive fails. It does not protect against deleted files, ransomware, controller failure or a fire. Keep a separate backup.
Which RAID level should I use?
For a few drives and data you can restore, RAID 5 or RAIDZ1 is common. With large drives or six or more of them, RAID 6 or RAIDZ2 is safer because a second drive can fail while the first is rebuilding. RAID 10 trades space for speed and fast rebuilds.
What happens with different sized drives?
Classic RAID treats every drive as the size of the smallest one, wasting the extra. Enter the size of the smallest drive here to see what you would actually get.
What are hot spares?
A hot spare is an extra drive that sits idle and takes over automatically when a drive fails. It adds no capacity, so the calculator takes spares out of the array.
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