RAID Capacity Calculator
Work out usable capacity, fault tolerance and efficiency for RAID 0, 1, 5, 6 and 10 before you buy the drives.
Your details
This calculator runs entirely on your device. Nothing you enter is uploaded, stored, or sold. How this works
Usable capacity
20.00 TB
About 18.19 TiB as your OS will report it
Includes your inputs, the full breakdown, and every row of the table.
Raw capacity
24.00 TB
Lost to redundancy
4.00 TB
Storage efficiency
83.3%
Fault tolerance
1 drive
- Drives
- 6 × 4.00 TB
- Raw total
- 24.00 TB
- Drives' worth of usable data
- 5.0
- Usable capacity
- 20.00 TB
- Usable as reported by the OS
- 18.19 TiB
| Level | Usable TB | Efficiency | Survives |
|---|---|---|---|
| RAID 0 | 24.00 | 100% | Nothing |
| RAID 1 | 4.00 | 16.7% | 5 drives |
| RAID 5 | 20.00 | 83.3% | 1 drive |
| RAID 6 | 16.00 | 66.7% | 2 drives |
| RAID 10 | 12.00 | 50% | 1 per pair |
What this means
- Drive manufacturers count a terabyte as 10^12 bytes while operating systems divide by 1024 three times. That is where the missing ~9% goes — nothing is actually wrong with the disk.
- RAID protects against drive failure, not against deletion, ransomware, controller faults or fire. It is an availability mechanism, not a backup.
- Single parity on large drives is increasingly discouraged. Rebuilds on multi-terabyte disks take many hours under full load, and a second failure during that window loses everything — consider RAID 6.
How the raid capacity calculation works
RAID trades capacity for the ability to survive a drive failure, and the exchange rate depends entirely on the level you choose. Striping gives you every byte you paid for and no protection at all. Mirroring gives you half. Parity levels sit in between and get more efficient the more drives you add, which is why RAID 5 and 6 dominate larger arrays while mirroring dominates small ones.
There are two numbers that surprise people. The first is the capacity your operating system reports, which is about 9% below the number on the box because manufacturers count in powers of ten and operating systems count in powers of two. The second is rebuild risk: when a drive in a parity array fails, replacing it forces every remaining drive to be read end to end, and that sustained load is exactly when a second, already-marginal drive tends to give out.
Frequently asked questions
Why is my array smaller than the drives I bought?
Two effects stack. Redundancy consumes whole drives' worth of space — one for RAID 5, two for RAID 6, half the array for RAID 10 — and then the decimal-to-binary conversion trims roughly another 9% from what the OS displays. A six-drive 4 TB RAID 5 array is 20 TB of usable space but shows as about 18.2 TiB.
Is RAID 5 still a reasonable choice?
It is increasingly hard to justify on large drives. Rebuilding a parity array reads every remaining disk in full, which takes many hours on multi-terabyte drives and applies exactly the kind of sustained stress that finds a second weak drive. RAID 6 survives that second failure, and the extra drive is cheap compared with losing the array.
Does RAID replace backups?
No, and treating it as though it does is the most expensive mistake in storage. RAID keeps you running through a hardware failure. It replicates deletions, corruption and ransomware faithfully to every disk, and it does nothing about theft, flood or a failed controller writing garbage. You still need copies somewhere else.
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Disclaimer. This calculator is provided for general information and educational purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Results are estimates based on the inputs you provide and the assumptions described above. Confirm any figure with a qualified professional before acting on it.