RAID 5, 6 and 10 Capacity Calculator

Compare raw and usable storage for common RAID layouts. The calculator assumes every active disk contributes only the capacity of the smallest disk, reports decimal terabytes, and subtracts designated hot spares before applying the RAID formula.

RAID capacity formulas

RAID 0 uses every active disk and provides no redundancy. RAID 1 mirrors data, so this calculator treats one disk’s capacity as usable even when the mirror contains more than two members. RAID 5 reserves one disk-equivalent for parity; RAID 6 reserves two. RAID 10 mirrors pairs and then stripes across them, producing half of the active raw capacity.

Why the smallest disk sets the limit

Traditional RAID groups divide data and parity into equal chunks. When disks differ in size, capacity beyond the smallest member’s size generally cannot participate in the array. Four disks labeled 12 TB, 12 TB, 8 TB, and 8 TB therefore behave like four 8 TB members for this calculation.

Decimal TB versus binary TiB

Drive manufacturers normally advertise decimal terabytes, where 1 TB is 1,000,000,000,000 bytes. Many operating systems report binary tebibytes, where 1 TiB is 1,099,511,627,776 bytes. An array advertised as 24 TB will appear smaller when software reports TiB, before filesystem and metadata overhead.

Fault tolerance is not a guarantee

RAID 10 may survive more than one failure when failed disks are in different mirror pairs, but can fail after two losses in the same pair. Rebuild time, unreadable sectors, controller behavior, and enclosure failures affect real outcomes. Keep separate, tested backups.

Frequently asked questions

How much usable space does RAID 5 provide?

RAID 5 usable capacity is the capacity of the smallest disk multiplied by one fewer than the number of active disks.

Is RAID a backup?

No. RAID can improve availability after some disk failures, but it does not protect against deletion, corruption, theft, or a failed enclosure.