SoupCalc

RAID Raw and Usable Capacity Calculator

RAID Raw and Usable Capacity Calculator

This tool estimates nominal raw capacity and capacity available after the redundancy overhead assumed for a selected RAID level. Enter an integer disk count, the equal capacity assigned to each disk, and whether that capacity is expressed in GB or TB. The output stays in the selected unit and does not convert decimal drive labels into binary GiB or TiB.

The estimate is about array layout, not formatted filesystem space. Controllers, metadata, spare disks, filesystems, over-provisioning, and vendor conventions can reduce the capacity shown by an operating system.

Method and formulas

With n equal-capacity disks:

  • RAID 0: usable = n × disk size; minimum 2 disks; no redundancy.
  • RAID 1: usable = 1 × disk size; minimum 2 disks; this calculator assumes every entered disk mirrors the same data.
  • RAID 5: usable = (n − 1) × disk size; minimum 3 disks.
  • RAID 6: usable = (n − 2) × disk size; minimum 4 disks.
  • RAID 10: usable = n ÷ 2 × disk size; at least 4 disks and an even disk count.
  • RAID 50: usable = (n − 2) × disk size; at least 6 disks and an even count, assuming exactly two equal RAID 5 groups striped together.
  • RAID 60: usable = (n − 4) × disk size; at least 8 disks and an even count, assuming exactly two equal RAID 6 groups striped together.

Raw capacity is always n × disk size. Invalid disk counts and unsupported topologies are rejected instead of returning a plausible-looking zero.

Worked example

For six 2 TB disks, RAID 50 produces 8 TB usable under this tool's two-group assumption. The two RAID 5 groups each spend one disk-equivalent on parity:

(6 − 2) × 2 TB = 8 TB usable

Raw capacity is 6 × 2 TB = 12 TB. The 4 TB difference is redundancy overhead under this topology, not a backup copy that can be restored independently.

How to interpret the result

“Usable” means nominal array capacity before controller and filesystem overhead. If disks differ in size, most conventional RAID layouts use only the capacity of the smallest disk on every member; calculate conservatively using that size.

Fault tolerance depends on which disks fail and on the nested layout. For example, RAID 10 may survive more than one disk failure if the failed disks are in different mirror pairs, but it can fail after two losses in the same pair. This calculator reports capacity, not failure probability or rebuild safety.

Capacity and data-safety limits

  • RAID is not a backup. It does not protect against deletion, corruption, malware, theft, fire, controller faults, or mistakes replicated across the array. Maintain tested, independent backups appropriate to the data.
  • RAID 0 has no disk-failure tolerance. One member failure can make the whole array unavailable.
  • Rebuilds stress surviving disks and expose the array to additional failure risk. Capacity alone is not enough to select a level for important data.
  • Vendor implementations differ, especially for nested levels, distributed spare space, declustering, and mixed disk sizes. Confirm topology and reported capacity in the controller documentation before purchase or migration.

Sources

Editorial record

  • Author: SoupCalc Editorial Team
  • Method review: SoupCalc Storage Engineering Review
  • Last reviewed: August 10, 2026
  • Review scope: supported levels, disk-count validation, nested-array assumptions, capacity examples, unit semantics, and backup warnings