RAID calculator
Usable capacity of a RAID array, its failure tolerance and the efficiency of each level.
Fill in the fields and the result will appear here automatically.
Compare ideal usable capacity for five RAID models with equal disks. RAID1 here stores a complete copy on each of n disks: usable space is half of raw capacity only when n=2. RAID10 uses conventional paired mirrors; the tolerated count is guaranteed independently of a fortunate failure distribution. Filesystem capacity and specific controller requirements need separate checks.
How it works
Formula and logic
For n equal disks of S decimal TB, raw capacity is nS. RAID0 gives nS with no redundancy; RAID1 gives S as n complete copies; RAID5 gives (n−1)S; RAID6 gives (n−2)S; RAID10 gives nS/2 for even n≥4 with paired mirrors. This tool’s minima are one disk for level 0 (a degenerate unstriped case), two for 1, three for 5 and four for 6. Guaranteed tolerable failures for a healthy synchronized array are respectively 0, n−1, 1, 2 and 1. These are not data-loss probabilities.
Example
RAID 5 built from six 4 TB disks gives 20 TB usable out of 24 TB raw, or 83.33%.
Fields and units
- RAID level — list option
- Number of disks — disks
- Size of one disk — TB
How to use
- — Choose the array level.
- — Enter the number of disks — RAID 10 needs an even count.
- — Enter the size of a single disk in terabytes.
- — Disks are assumed identical: an array levels down to its smallest one.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- IBM Cloud: RAID mirroring, parity and limits NIST: decimal and binary prefixes
- Limitation
- Educational RAID layouts with equal synchronized disks. Actual implementations, metadata, rebuilds, latent read errors and backups are not modelled. Interface minima are not universal controller requirements.
FAQ
Which level suits a home storage box?
Choice depends on capacity, availability, writes, rebuilds and backups. This page compares ideal capacities and tolerated failure counts; it does not calculate rebuild probability or recommend a level from disk count.
What happens with disks of different sizes?
The array levels down to the smallest one: an 8 TB disk paired with a 4 TB disk contributes only 4 TB. That is why the calculation assumes identical disks.
Why does RAID 10 show only one tolerated failure?
That is the guaranteed figure. The array can survive half its disks if the failures land in different mirrors, but two failures inside one mirror destroy it at any size — a lucky distribution cannot be promised.
Does RAID replace a backup?
No. RAID protects against a disk failing, not against a deleted file, ransomware, fire or theft — all of which hit the whole array equally. A backup is a separate requirement.
Why does the manufacturer promise more terabytes than the system shows?
Manufacturers count a terabyte as 10¹² bytes while the system displays tebibytes of 2⁴⁰ bytes. The gap is about 9% and has nothing to do with the array level.