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RAID Calculator

Plan a storage array before you buy drives. Choose a RAID level, the number of drives and their size to see the usable capacity, how much goes to redundancy, the storage efficiency and how many drive failures the array survives.

Quick examples

RAID 5 needs 3 or more, RAID 6 needs 4 or more, RAID 10 needs an even number from 4.

With mixed sizes, enter the smallest drive.

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Usable capacity12 TB
Used for redundancy
4 TB
Raw capacity
16 TB
Storage efficiency
75%
As your operating system shows it
10.91 TiB
Drive failures survived
1 drive
Raw capacity
  • Usable
  • Redundancy

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    Results you calculate here are kept on this device so you can come back to them.

      Formula

      RAID 0: usable = N × drive size
      RAID 1: usable = 1 × drive size
      RAID 5: usable = (N − 1) × drive size
      RAID 6: usable = (N − 2) × drive size
      RAID 10: usable = (N ÷ 2) × drive size
      TiB = bytes ÷ 1,024⁴

      How to use it

      1. Choose the RAID level.
      2. Enter the number of drives; an invalid count for the level gives a blank result.
      3. Enter the capacity of one drive — the smallest, if they differ.
      4. Compare usable space and fault tolerance across levels.

      Worked examples

      RAID 5 with four 4 TB drives: 12 TB usable, one drive’s worth of parity

      Usable capacity
      12 TB
      Used for redundancy
      4 TB
      Raw capacity
      16 TB
      Storage efficiency
      75%
      As your operating system shows it
      10.91 TiB
      Drive failures survived
      1 drive

      RAID 10 with six 8 TB drives: 24 TB usable

      Usable capacity
      24 TB
      Used for redundancy
      24 TB
      Storage efficiency
      50%
      As your operating system shows it
      21.83 TiB
      Drive failures survived
      1 drive per mirror pair (up to 3 if each is in a different pair)

      RAID 6 with eight 10 TB drives: 60 TB usable

      Usable capacity
      60 TB
      Raw capacity
      80 TB
      Storage efficiency
      75%
      As your operating system shows it
      54.57 TiB
      Drive failures survived
      2 drives

      The levels in brief

      RAID 0 stripes data across all drives for speed and full capacity, with no protection. RAID 1 mirrors the same data on every drive. RAID 5 spreads one drive’s worth of parity across at least three drives and survives one failure. RAID 6 uses two drives’ worth across at least four and survives two. RAID 10 stripes across mirrored pairs: half the capacity, fast rebuilds, and it survives one failure per pair.

      Why the system shows less

      Drive makers count a terabyte as 1,000⁴ bytes; most operating systems report in tebibytes of 1,024⁴ bytes but often still write “TB”. A 12 TB volume appears as about 10.9. The file system takes a little more on top.

      RAID is not a backup

      RAID keeps an array running through a drive failure. It does nothing about accidental deletion, ransomware, file system corruption, fire or theft, all of which hit every drive at once. Keep a separate backup whatever level you choose.

      Questions people ask

      How much usable space do four 4 TB drives give in RAID 5?

      12 TB: one drive’s worth of the 16 TB raw capacity holds parity. Your system will show about 10.9 TiB.

      How many drives can fail in RAID 6?

      Any two at the same time without losing data.

      What is the difference between RAID 5 and RAID 10?

      With four 4 TB drives, RAID 5 gives 12 TB and survives one failure; RAID 10 gives 8 TB, survives one failure in each mirror pair, and rebuilds faster.

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