RAID Calculator: Usable Capacity & Fault Tolerance

N is the number of drives in the array, excluding hot spares. Usable capacity is N × drive size for RAID 0 and JBOD; one drive’s capacity for RAID 1; (N − 1) × drive size for RAID 5; (N − 2) × drive size for RAID 6; N / 2 × drive size for RAID 10; (N − groups) × drive size for RAID 50; and (N − 2 × groups) × drive size for RAID 60. For example, 4 drives of 4 TB provide 12 TB with RAID 5 and 8 TB each with RAID 6 and RAID 10. The calculator assumes equally sized drives. If you combine different sizes, each drive in traditional RAID levels usually contributes only as much as the smallest drive, so plan using that size.

RAID 5 requires at least 3 drives, RAID 6 at least 4, and RAID 10 at least 4 with an even drive count. RAID 0 and RAID 1 start at 2 drives; JBOD starts at 1. RAID 50 requires 3 drives per group and RAID 60 requires 4, meaning 6 and 8 respectively with 2 groups. Hot spares do not count. If there are too few drives, the calculator shows the required minimum.

RAID 0 and JBOD tolerate no drive failures; RAID 5 tolerates one, RAID 6 two, and RAID 1 all but one. RAID 10 is guaranteed to survive one failure and can survive up to N / 2 in the best case, as long as one drive in each mirror pair remains intact. RAID 50 is guaranteed to survive one failure and can survive up to one per group; RAID 60 is guaranteed to survive two and can survive up to two per group. The calculator shows the guaranteed and best-case values separately.

RAID 6 uses the capacity of one more drive but tolerates two failures instead of one: 4 × 4 TB yields 8 TB rather than 12 TB. Rebuilding is the key concern. After a failure, RAID 5 must read all remaining drives in full. With large drives, this takes hours to days, and a second failure or read error can destroy the entire array. For RAID 5 drives of 4 TB or more, the calculator therefore notes that RAID 6 or RAID 10 is usually safer.

RAID 10 is a common choice for databases, virtualization, and game servers: it is robust, but mirroring uses half the capacity. RAID 6 suits large arrays for archives and backups; RAID 5 is a compromise for a small number of smaller drives. With two drives, your options are RAID 1 mirroring or RAID 0 with no fault tolerance. RAID 0 is suitable only for data you can recreate at any time; the calculator warns about its lack of redundancy.

A hot spare is an extra drive installed in the server that does not store data. If a drive in the RAID fails, the hot spare automatically takes over and the RAID rebuilds. It does not count toward usable capacity: in the calculator, it is included in the total drive count but not in the array. A hot spare provides no benefit for RAID 0 or JBOD because they have no redundancy; the calculator points this out.

Drive manufacturers use decimal units (1 TB = 1,000,000,000,000 bytes), while operating systems often display tebibytes (1 TiB = 1,099,511,627,776 bytes), usually without saying “tebi”. A 4 TB drive therefore appears as about 3.64 TiB, and 12 TB of usable capacity as about 10.91 TiB. That is why the calculator also shows the TiB value. File system overhead and reserved space can use another few percent, depending on the file system.

No. RAID protects against individual drive failures, but not accidentally deleted files, malware, software errors, or failure of the controller or entire server. You still need a Backup on a separate system, even with RAID.