How Many Drives Can Fail in Each RAID Level?
Fault tolerance is how many drives an array can lose without losing data. Parity levels give a fixed number. Mirrored and grouped levels give a guaranteed minimum, plus a best case that depends on which drives fail.
The short answer
- RAID 0: none.
- RAID 1: all but one drive.
- RAID 5: 1 drive.
- RAID 6: any 2 drives.
- RAID 10: at least 1; up to one per mirrored pair.
- RAID 50: at least 1; up to one per RAID 5 group.
- RAID 60: at least 2; up to two per RAID 6 group.
Identical drives; decimal TB as drives are sold, binary TiB as most operating systems show it; before filesystem overhead. Hot spares are extra drives and add no capacity.
Fault tolerance with 16TB drives
| Level | Array | Usable | Failures always survived | Best case |
|---|---|---|---|---|
| RAID 0 | 4 × 16TB | 64TB | 0 | 0 |
| RAID 1 | 2 × 16TB | 16TB | 1 | 1 |
| RAID 5 | 6 × 16TB | 80TB | 1 | 1 |
| RAID 6 | 8 × 16TB | 96TB | 2 | 2 |
| RAID 10 | 8 × 16TB | 64TB | 1 | 4 |
| RAID 50 | 12 × 16TB, 2 groups | 160TB | 1 | 2 |
| RAID 60 | 16 × 16TB, 2 groups | 192TB | 2 | 4 |
Guaranteed versus best case
The guaranteed number holds whichever drives fail. The best case needs the failures to land in different mirrored pairs (RAID 10) or different groups (RAID 50/60). Two failures in the same RAID 10 pair, or three in the same RAID 60 group, lose the array.
The rebuild window
After a failure, the controller rebuilds the lost drive from the others, reading them in full. Until it finishes, RAID 5 and each RAID 50 group have no protection left, while RAID 6 and each RAID 60 group still have one drive of protection. Larger drives take longer to rebuild.
Hot spares do not raise the count
A hot spare starts the rebuild automatically, which shortens the time an array runs degraded. It does not let the array survive more simultaneous failures.
16TB drives for this array
In-stock 16TB drives from our catalogue, with the price for the 9 drives in the example above. Hot spares are extra.
- SHD7B06079 - Lenovo 16TB SATA 3.5-Inch Hot-Swap Hard Drive (512e, SED) $247.50 each · 9 drives $2,227.50
- Seagate ST16000NM010G Exos X16 16TB 7200RPM 12Gb/s SAS 256MB Cache 512e Enterprise $429.47 each · 9 drives $3,865.23
- ST16000NM003G - Seagate Exos X16 16TB 7200RPM SATA 6Gb/s 256MB Cache 512e Sed 3.5-inch $447.37 each · 9 drives $4,026.33
- ST16000NM002G - Seagate Exos X16 16TB 7200RPM SAS 12Gb/s 256MB Cache 512e 3.5-inch Hard $450.53 each · 9 drives $4,054.77
- ST16000VN001 - Seagate IronWolf Nas 16TB 7200RPM SATA 6Gb/s 256MB Cache 3.5-inch Hard $468.42 each · 9 drives $4,215.78
- Seagate ST16000NM007H Exos X24 16TB 7200RPM 12Gb/s SAS 512MB Cache 512e 3.5-Inch Hard $478.95 each · 9 drives $4,310.55
- Toshiba MG08ACA16TE 16TB 7200RPM 6Gb/s SATA 512MB Cache 512e 3.5-Inch Hard Drive $468.42 each · 9 drives $4,215.78
- Seagate ST16000NM004G Exos X16 16TB 7200RPM 12Gb/s SAS 256MB Cache Self-Encrypting Drives $500.00 each · 9 drives $4,500.00
Frequently asked questions
How many drives can fail in RAID 6?
Any two. RAID 6 keeps two drives’ worth of parity, so it also survives a second failure while rebuilding from the first.
Can RAID 10 survive two drive failures?
Yes, if the two failed drives are in different mirrored pairs. If both drives of one pair fail, the array is lost.
How many drives can fail in RAID 60?
At least two, and up to two in each RAID 6 group. Three failures in the same group lose the array.
Does a hot spare increase fault tolerance?
No. It shortens the time the array spends degraded by starting the rebuild straight away, but the number of simultaneous failures the level survives stays the same.
RAID calculators by level
- RAID calculator (all levels)
- RAID 0 calculator
- RAID 1 calculator
- RAID 5 calculator
- RAID 6 calculator
- RAID 10 calculator
- RAID 50 calculator
- RAID 60 calculator
RAID planning guides
- RAID Hot Spare: What It Does and How It Affects Capacity
- TB vs TiB: Why a 24TB Drive Shows 21.8 TiB
- RAID Write Penalty by Level
- Best RAID Level for 2, 3, 4, 5, 6 or 8 Drives
- RAID 50 vs RAID 60
- RAID 6 vs RAID 60
Related
Updated 2026-10-22.