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RAID Capacity Calculator

Find usable RAID capacity by level.

Result

How it is calculated

The tool lists usable space for four common RAID levels from the number of disks n and the size of each disk s in TB, assuming all disks are identical. RAID 0: n × s (striping, no redundancy). RAID 5: (n − 1) × s (one disk of parity). RAID 6: (n − 2) × s (two disks of parity). RAID 10: ⌊n ÷ 2⌋ × s (mirrored pairs). The disk count is rounded to a whole number, values below zero are floored at 0 TB, and the size must be positive. A level that cannot be built with that many disks shows "—": RAID 5 needs at least 3 disks, RAID 6 at least 4, and RAID 10 an even number of at least 4. Real capacity is a bit lower because of formatting and because drive makers count TB as 1000 GB.

Worked example

Four disks of 8 TB each. RAID 0: 4 × 8 = 32.00 TB, with no protection. RAID 5: (4 − 1) × 8 = 24.00 TB, survives one failed disk. RAID 6: (4 − 2) × 8 = 16.00 TB, survives two. RAID 10: 2 pairs × 8 = 16.00 TB, survives one failure per pair.

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Common mistakes

  • Mixing disk sizes. The array treats every disk as the size of the smallest one, but this tool assumes equal disks, so enter the smallest size.
  • Entering a disk count that is too small for the level. RAID 5 needs at least 3 disks, RAID 6 at least 4 and RAID 10 an even number of at least 4; for fewer disks the tool shows "—" for that level.
  • Treating RAID as a backup. It protects against disk failure, not against deletion, ransomware or a controller fault.
  • Expecting RAID 10 to give the same space as RAID 5. It always keeps half the raw capacity.
  • Forgetting that the OS shows less than the label, because TB is counted in decimal and TiB in binary.

About this calculator

Usable capacity depends on how a RAID level distributes data and redundancy across n disks of a given size: RAID 0 (striping, no redundancy) gives the full n × disk size but no fault tolerance; RAID 5 (single parity) gives (n − 1) × disk size, tolerating one disk failure; RAID 6 (double parity) gives (n − 2) × disk size, tolerating two simultaneous failures; and RAID 10 (mirrored stripes) gives (n / 2) × disk size, since every disk is mirrored.

IT sysadmins and storage engineers use this when designing NAS or SAN arrays to balance capacity, performance, and fault tolerance for a given budget, data-center architects use it when specifying storage for enterprise servers, and home-lab and NAS hobbyists (running Synology, TrueNAS, or Unraid setups) use it to decide how many drives and what RAID level gets them the storage and redundancy they need.

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Comments (1)

  • Editorial team

    These are raw usable sizes only. Real arrays lose a little more to formatting, file system overhead and TB versus TiB differences, so a drive sold as 8 TB shows less in the operating system. A level the disk count cannot support shows a dash: RAID 5 needs at least 3 disks, RAID 6 at least 4, and RAID 10 an even number of at least 4. Which array layout are you weighing, and why?

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