What it does
Calculates the usable storage capacity, storage efficiency, and fault tolerance of a RAID array based on the number of disks, the capacity of each disk, and the RAID level. Useful when planning a NAS, server, or storage pool and you need to know how much usable space you'll actually end up with — and how many drives can fail before you lose data.
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How it works
For each RAID level, the calculator applies the standard formula for usable capacity and minimum/maximum drive-failure tolerance:
| RAID | Min disks | Usable capacity | Fault tolerance |
|---|---|---|---|
| RAID 0 | 2 | N × size (100% efficiency) | 0 disks — no redundancy |
| RAID 1 | 2 | size (capacity of one disk) | N − 1 disks (as long as 1 survives) |
| RAID 5 | 3 | (N − 1) × size | 1 disk |
| RAID 6 | 4 | (N − 2) × size | 2 disks |
| RAID 10 | 4 (even) | (N / 2) × size | 1 disk per mirrored pair |
- You enter the RAID level, the number of disks in the array, and the capacity of a single disk (all disks are assumed to be the same size, which is the standard, recommended RAID configuration).
- The tool validates the disk count against the minimum required disks for that RAID level (and, for RAID 10, that the disk count is even).
- It then computes:
- Total raw capacity —
N × size. - Usable capacity — using the formulas above.
- Storage efficiency —
usable ÷ raw, as a percentage. - Fault tolerance — how many disks can fail simultaneously before data loss (worst case for RAID 1/10 with more than 2 disks).
Notes
- This is a theoretical calculation based on raw decimal capacity (1 TB = 1000 GB). Real-world usable space is usually slightly lower due to filesystem overhead, drive formatting (binary vs. decimal capacity), and RAID controller/metadata reservations.
- RAID 10 requires an even number of disks (pairs of mirrors striped together). Odd counts are rejected.
- RAID is not a backup. Even RAID 6 (double parity) only protects against drive failure — it does not protect against accidental deletion, ransomware, fire, or theft.
- All processing happens entirely in your browser — nothing is sent anywhere.