192 RAID cells — four 4 TB disks in RAID 5 make 12 TB, and show up as 10.91 TiB
You do not need a separate formula per level. Knowing how many disks each group spends on parity gives you the rest in one line: usable = total − parity × groups. RAID 5 spends one in a single group, so n−1; RAID 6 spends two, so n−2; RAID 50 and 60 run several such groups. 38 of the cells are impossible: three disks will not make a RAID 6, an odd count will not make a RAID 10, and seven disks will not make a RAID 50.
Usable disks
Disks
| 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| RAID 0 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 |
| RAID 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |
| RAID 5 | · | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 |
| RAID 6 | · | · | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 |
| RAID 10 | · | · | 2 | · | 3 | · | 4 | · | 5 | · | 6 | · | 7 | · | 8 | · | 9 | · | 10 | · | 11 | · | 12 | · |
| RAID 50 | · | · | · | · | 4 | · | 6 | 6 | 8 | · | 10 | · | 12 | 12 | 14 | · | 16 | · | 18 | 18 | 20 | · | 22 | 20 |
| RAID 60 | · | · | · | · | · | · | 4 | · | 6 | · | 8 | · | 10 | 9 | 12 | · | 14 | · | 16 | 15 | 18 | · | 20 | 15 |
| JBOD | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 |
Usable = total − parity × groups. No need for a separate formula per level. With the 38 impossible combinations and the TB-versus-TiB gap.
RAID capacity
- RAID 0Minimum disks 2
Data is simply spread across the disks. Capacity and speed scale with the count, but one failure loses everything.
- RAID 1Minimum disks 2
Every disk holds the same data. It survives down to a single disk, but only one disk’s worth is usable.
- RAID 5Minimum disks 3
One disk’s worth goes to parity. A single failure can be rebuilt from the rest.
- RAID 6Minimum disks 4
Two disks’ worth go to parity, so two simultaneous failures are survivable — a second disk failing during a rebuild does happen.
- RAID 10Minimum disks 4
Disks are mirrored in pairs, and the pairs are striped together. Half is usable and rebuilds are fast.
- RAID 50Minimum disks 6
Several RAID 5 groups striped together. Each group spends one disk on parity, so more groups means less capacity and more failures survived.
- RAID 60Minimum disks 8
Several RAID 6 groups striped together. Each group spends two disks on parity.
- JBODMinimum disks 2
The disks are simply concatenated into one volume. No parity, so no failure is survived — but a dead disk takes only its own data with it.
How the groups are split
For RAID 50 and 60, how many groups you use is the real decision. Fewer groups spend less on parity and leave more capacity; more groups leave one (or two) spare disks per group, so more failures are survivable when they land well. The guaranteed figure does not change either way — two failures inside one group end it regardless.
TB on the label, TiB on the screen
One TB on the box is 10¹² bytes, while the operating system counts in units of 2⁴⁰ bytes. The names look alike but the units differ, so the same disk reads about 9% smaller. Nothing went missing — the ruler changed.
The rebuild is the dangerous part
When a single-parity array loses a disk, you put a new one in and every remaining disk is read end to end to rebuild it. That is precisely when the survivors work hardest and longest. The bigger and more numerous the disks, the longer the rebuild — days, sometimes — and a second failure during it ends the array. This is why RAID 6 or 10 is preferred over RAID 5 on large arrays.
RAID is not a backup
RAID protects against exactly one thing: a disk dying. A file deleted by mistake, a file overwritten, ransomware, a failed controller, fire or theft all propagate to every disk at once — a mirror deletes just as faithfully as it writes. A backup is a separate copy on separate hardware, and RAID is not doing that job.
How to read it
- Usable = total − parity × groups. Every level is that one line.
- RAID 5 spends one parity disk in a single group, so n−1; RAID 6 spends two, so n−2.
- RAID 50 and 60 run several such groups striped together, so the parity is multiplied by the group count.
- RAID 1 holds one copy on every disk, so one disk’s worth; RAID 10 mirrors in pairs, so half.
- Groups must be equal in size, so the group count can only be a divisor of the disk count.
- TB on the box and TiB on the screen are different units — about 9% apart.
Common questions
Q. RAID 5 or RAID 6?
The bigger and more numerous the disks, the more it points to RAID 6. When RAID 5 loses a disk, rebuilding reads every remaining disk end to end, which on modern drives takes days. A second failure during that window ends the array — and RAID 6 is exactly what survives it.
Q. What if the disks are different sizes?
Everything is levelled down to the smallest one. Put an 8 TB disk alongside three 4 TB disks and only 4 TB of it is used; the other 4 TB sits idle. This chart assumes all disks are the same size.
Q. Why is the capacity less than the label says?
The label’s 1 TB is 10¹² bytes, while the operating system counts in units of 2⁴⁰ bytes. Similar names, different units — about 9% apart. Filesystem overhead takes a little more on top of that.
Q. If I have RAID, do I still need backups?
Yes. RAID protects against exactly one thing: a disk dying. A file deleted by mistake, ransomware, fire or theft reach every disk equally — a mirror copies the deletion just as faithfully as the data.