What to enter
For RAID 10, 50, and 60, enter the drives in the order you'll group them. In RAID 10, drives 1 and 2 form the first mirror pair, drives 3 and 4 the second, and so on. RAID 50 and 60 fill each group the same way, so put drives of the same size next to each other.
The address bar keeps your drive list, so you can bookmark or share the result.
How to read the result
Usable capacity is the space the array gives you before a filesystem goes on it. It shows in TiB. Switch to TB to compare it with drive labels.
The bar splits the raw space of all drives into usable space, redundancy, and size mismatch. Redundancy is the mirror copies or parity that let the array survive a failed drive. Size mismatch is space above the smallest drive in a group, which the array can't use.
Failure tolerance first says how many drives can fail, whichever ones they are. For RAID 10, 50, and 60, it then gives a higher number that holds only when the failed drives sit in different groups.
Four drives of 4, 4, 8, and 8 TB in RAID 5
Example
Usable capacity is 10.91 TiB, which is 12 TB. Redundancy takes 3.64 TiB. Size mismatch takes 7.28 TiB, because RAID 5 uses only 4 TB of each 8 TB drive. Any one drive can fail.
How much usable space does each RAID level give?
With equal drives, usable space is a whole number of drives. N is the number of drives, S is the size of the smallest one, and G is the number of groups.
| Level | Min. drives | Usable space | Drives that can fail |
|---|---|---|---|
| RAID 0 | 2 | N × S | None |
| RAID 1 | 2 | S | N - 1 |
| RAID 5 | 3 | (N - 1) × S | 1 |
| RAID 6 | 4 | (N - 2) × S | 2 |
| RAID 10 | 4, even | N ÷ 2 × S | 1, or one per pair |
| RAID 50 | 6 | (N - G) × S | 1, or one per group |
| RAID 60 | 8 | (N - 2G) × S | 2, or two per group |
| Linear or JBOD | 1 | All drives added up | None |
What is the RAID write penalty?
It's the number of disk operations one small write causes. To change one small block in RAID 5, the array reads the old data and parity, then writes both.
| Level | Disk operations per small write |
|---|---|
| RAID 0, Linear, JBOD | 1 |
| RAID 1 | 1 per drive |
| RAID 10 | 2 |
| RAID 5 and RAID 50 | 4 |
| RAID 6 and RAID 60 | 6 |
It explains why parity layouts are slower at small random writes. It isn't a speed measurement, because caches and full-stripe writes change real performance. Dell's RAID reference lists the same counts.
Can you mix drive sizes in RAID?
Yes, but every drive in a group gives only as much space as the smallest drive in that group. A mirror of a 4 TB and an 8 TB drive holds 4 TB. Linear and JBOD are the exception: they join drives end to end, so every drive counts in full, with no redundancy.
If you want mixed drives with redundancy and little waste, look at Synology SHR in the SHR calculator or Unraid in the Unraid calculator. Linux md RAID 0 can also use part of the extra space on larger drives. This calculator doesn't model that.
Does drive order matter in RAID 10?
Yes. Each mirror pair is limited by its smaller drive, so pairing drives of the same size keeps the most space. The same four drives give different results:
| Order entered | Mirror pairs | Usable | Size mismatch |
|---|---|---|---|
| 4, 4, 8, 8 TB | 4 + 4 and 8 + 8 | 10.91 TiB (12 TB) | 0 |
| 4, 8, 4, 8 TB | 4 + 8 and 4 + 8 | 7.28 TiB (8 TB) | 7.28 TiB |
RAID 50 and RAID 60 work the same way inside each group.
How many drives can fail in RAID 10, 50, or 60?
Any one drive can fail in RAID 10 and RAID 50, and any two in RAID 60. More can fail only if they land in different groups.
An eight-drive RAID 10 survives four failed drives if each comes from a different mirror pair. It doesn't survive losing both drives of one pair. In RAID 50 and 60, losing one whole group loses the array.
Why does my array show less space than the calculator?
Drives are sold in TB, but most computers count in TiB. A TB is 1,000,000,000,000 bytes and a TiB is 1,099,511,627,776 bytes, so a 4 TB drive is 3.64 TiB. Windows shows TiB values but labels them TB, as Seagate explains. macOS counts in TB, like the label, according to Apple.
The result is also the size of the array before anything is on it. Partitions, the filesystem, reserved space, and snapshots take more. A NAS also keeps a system partition on every drive.
Should you use RAID 5 or RAID 6?
The difference shows when a drive fails. RAID 5 has no protection left while it rebuilds, so a second failure loses the array. RAID 6 can still lose one more drive.
A rebuild reads every remaining drive, so larger drives take longer to rebuild. The rebuild calculator estimates how long your array would stay exposed.
Is RAID a backup?
No. RAID keeps the array running when a drive fails. It doesn't protect you from deleted files, ransomware, theft, fire, or a fault that corrupts the whole array. Keep a separate copy and test that it restores. The 3-2-1 backup calculator helps you size one.
What should you check before building the array?
Check that your controller or operating system supports the layout. Some vendors use JBOD to mean separate disks, not one combined volume. ZFS RAIDZ uses different math, so use the ZFS capacity calculator for a ZFS pool, or the storage layout advisor to compare layouts for your drives and the failures you need to survive.