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RAID Usable Capacity and Fault Tolerance Calculator

Web

Everything runs locally in your browser — nothing is uploaded

Output

Enter the level, disk size and disk count to get the usable capacity, how many disks can fail, the minimum number of disks, and whether the layout is valid. Two things that are easy to get wrong each get their own line: RAID 50/60 is "N groups × M disks each", so the disk count must divide by the group size; and TB is 10¹² while TiB is 2⁴⁰ — a drive sold as 4 TB shows about 3.64 TiB in the OS, a 10% gap that has nothing to do with RAID. Everything is computed locally in your browser; nothing leaves the device.

Features

  • Covers RAID 0 / 1 / 5 / 6 / 10 / 50 / 60 and JBOD
  • Gives usable capacity, tolerated failures, minimum data disks and whether the layout is valid
  • Checks even disk counts for RAID 10, minimums for 5/6, and divisibility for 50/60
  • Hot spares are removed from the data disks and never counted as usable capacity
  • Shows both vendor TB and OS TiB (4 TB shows about 3.64 TiB)
  • A one-line read/write performance note for every level
  • Invalid layouts raise a clear error instead of a plausible-looking capacity

How to use

  1. Pick a RAID level
  2. Enter the disk size (GB) and the total disk count
  3. Add hot spares if needed, and the group size for RAID 50/60
  4. Capacity, tolerance and validity update live

FAQ

With 12 disks in RAID 50, what group size works?
It must divide 12, and each group needs at least 3 disks: groups of 3 give 4 groups, groups of 4 give 3 groups, groups of 6 give 2 groups. Groups of 5 do not divide — a controller would either reject it or waste two disks, so this tool raises an error.
Why does a 4 TB drive show only 3.64 TiB?
Vendors count in base 10 (4 TB = 4×10¹² bytes) while the OS displays in base 2 (÷2⁴⁰ = 3.64 TiB). Neither is wrong — one word, "TB", is shared by two bases. RAID is not eating your capacity; the gap is unrelated to whether you use RAID at all.
What is the minimum disk count for RAID 5 and RAID 6?
RAID 5 needs at least 3 disks (2 data + 1 parity) and RAID 6 at least 4 (2 data + 2 parity). Below that there is no redundancy, and the tool raises an error.
Is the fault tolerance figure a guarantee?
Yes. RAID 10 guarantees one disk; in the best case one disk per mirrored pair can fail, but that depends on which disks fail, so it is only noted, not counted as the promised tolerance. RAID 50/60 tolerance is one/two disks per group.

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