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RAID and NAS Capacity Calculator

Choose your disks and RAID level to see how much usable space you actually get, how much goes to redundancy, and how many disks the array can lose.

🔒 Runs entirely in your browser. Nothing you type is sent anywhere.

Usable space is shown in both decimal (TB) and binary (TiB) units. Most operating systems report binary, so a 4 TB disk shows as about 3.64 TiB. This is an estimate: real arrays lose a little more to formatting and metadata.

How to use it

  1. Enter how many disks you have and the size of each one.
  2. Pick the RAID level, or Synology SHR if you run a Synology NAS.
  3. Read off the usable capacity, how much is lost to redundancy, and how many disks the array survives losing.

Which RAID level

  • RAID 0: all capacity, no redundancy. One failed disk loses everything. Fine for scratch data, not for anything you care about.
  • RAID 1: a straight mirror of two disks. Simple and safe, but you only keep half the space.
  • RAID 5: one disk of parity. Good balance for three or more disks, but a second failure during a rebuild loses the array.
  • RAID 6: two disks of parity. The safer choice for large disks, where rebuilds take a long time.
  • RAID 10: mirrored pairs that are then striped. Fast and resilient, but you lose half the space.
  • Synology SHR: Synology’s own layout. It behaves like RAID 5 (or RAID 6 with the two-disk option) but uses more of the space when your disks are different sizes.

Why the numbers look smaller than the label

Disk makers count a terabyte as 1,000,000,000,000 bytes, but most operating systems count in binary, where a tebibyte is 1,099,511,627,776 bytes. So a 4 TB disk shows as about 3.64 TiB before any RAID overhead. The calculator shows both.

FAQ

Is RAID a backup? No. RAID keeps a server running through a disk failure, but it does not protect against deletion, ransomware, fire or theft. Keep a separate backup as well.

Can I mix disk sizes? In classic RAID each disk is used only up to the size of the smallest. Synology SHR is the exception and is designed for mixed sizes.

Is anything sent to a server? No. The maths runs in your browser.