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RAID Capacity Calculator

Work out how much usable storage a RAID array actually gives you once mirroring or parity has taken its share. Enter the size of each disk, how many disks you have and the RAID level (0, 1, 5, 6 or 10) and the tool returns the usable capacity, the raw total, the capacity spent on redundancy and the storage efficiency.

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How to use this calculator

Enter Values

Enter your values and press Calculate to see results.

Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.

How to use this calculator

  1. Enter the size of a single disk in TB (all disks are assumed the same size — RAID uses the smallest disk's size).
  2. Enter the number of disks in the array and the RAID level as a number: 0, 1, 5, 6 or 10.
  3. Read the usable capacity, plus how much is lost to redundancy and the storage efficiency as a percentage.

How it works

Raw capacity is simply the disk size times the number of disks. Each RAID level then keeps a different amount usable: RAID 0 stripes with no protection so all of it is usable (n × S); RAID 1 and RAID 10 mirror, so half is a copy (usable = (n / 2) × S); RAID 5 dedicates the equivalent of one disk to parity (usable = (n − 1) × S) and RAID 6 two disks (usable = (n − 2) × S). Storage efficiency is usable ÷ raw × 100. Each level has a minimum disk count — RAID 5 needs at least 3, RAID 6 at least 4, RAID 10 at least 4 and an even number, and any mirror needs an even count — and the calculator rejects invalid combinations.

Worked example

Worked example. Six 4 TB disks in RAID 6: raw = 6 × 4 = 24 TB, two disks go to parity so usable = (6 − 2) × 4 = 16 TB, 8 TB is lost to redundancy and efficiency is 16 ÷ 24 × 100 = 66.67%.

Common mistakes

  • Confusing usable with formatted capacity — filesystem overhead and decimal-TB vs binary-TiB labelling (about 9% less) mean the OS will report noticeably less than the usable figure here.
  • Assuming RAID replaces backups — RAID survives disk failures but not accidental deletion, ransomware, controller faults or fire; you still need real backups.
  • Forgetting hot/cold spare disks — leave one or more disks as spares outside the array so it can rebuild automatically, rather than counting every disk as capacity.

Frequently asked questions

Does RAID 1 with more than two disks change the maths?

This tool treats RAID 1 as mirror pairs, so usable = (n / 2) × S and it requires an even disk count. A true n-way mirror (three or more copies of one disk) would give usable = S but with more redundancy; that is an uncommon configuration and is modelled here as RAID 10 / paired mirrors instead.

Why is the OS capacity lower than the usable figure?

Two reasons: the filesystem itself reserves space for metadata, journals and reserved blocks, and drive makers sell disks in decimal TB (10^12 bytes) while operating systems count in binary TiB (2^40 bytes), which alone is about a 9% gap. Treat the usable figure as an upper bound.

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