What this RAID volume calculator does
This RAID capacity calculator estimates how much usable storage you get after applying RAID overhead, hot spares, and optional filesystem overhead. It is designed for quick planning when sizing a NAS, home lab server, media archive, backup target, or small business file server.
RAID can improve availability and, depending on level, performance. But every RAID type trades capacity for different levels of fault tolerance. Knowing your usable space ahead of time helps you avoid buying too few drives—or overbuilding unnecessarily.
Quick RAID capacity reference
| RAID Level | Minimum Drives | Usable Capacity Formula* | Fault Tolerance |
|---|---|---|---|
| RAID 0 | 1 | N × D |
0 drives |
| RAID 1 | 2 | 1 × D (single mirrored set) |
Up to N-1 (if one member survives) |
| RAID 5 | 3 | (N - 1) × D |
1 drive |
| RAID 6 | 4 | (N - 2) × D |
2 drives |
| RAID 10 | 4 | (N / 2) × D (even N) |
At least 1, potentially more |
*N = active data drives in the array (after removing spares), D = capacity per drive.
How to use this NAS RAID calculator
1) Choose your RAID level
Select RAID 0, 1, 5, 6, or 10 based on your needs for speed, resilience, and capacity efficiency. If you are not using RAID, you can choose JBOD to estimate simple pooled volume size.
2) Enter drive count and size
Input the total number of installed disks and each disk's capacity in GB or TB. For best accuracy, use the actual marketed drive size and remember that operating systems may report binary units (GiB/TiB) slightly lower than decimal sizes.
3) Add hot spares and overhead
Hot spare disks do not contribute to normal usable capacity, but they improve recovery time during failure events. Filesystem overhead (metadata, reserved blocks, snapshots) further reduces available space.
Choosing the right RAID level
- RAID 0: Maximum capacity and speed, no protection. One disk failure loses the full array.
- RAID 1: Best for simple two-disk redundancy and straightforward rebuilds.
- RAID 5: Good balance of capacity and protection for general file storage.
- RAID 6: Better for larger arrays where dual-disk failure risk is non-trivial.
- RAID 10: Excellent random I/O and rebuild behavior, with 50% capacity efficiency.
Common RAID planning mistakes
- Confusing RAID with backup: RAID improves uptime, but accidental deletion and ransomware still require real backups.
- Ignoring rebuild risk: Larger drives take longer to rebuild, increasing exposure windows.
- Mixing unlike drives: Arrays typically size to the smallest drive, wasting larger disks.
- Skipping growth headroom: Plan for snapshots, retention, and data growth over 12-24 months.
Practical example
Suppose you have 6 drives of 12 TB each, using RAID 6, with 1 hot spare and 3% filesystem overhead.
- Installed raw: 72 TB
- Active drives in array: 5 (one is spare)
- RAID 6 usable before filesystem: (5 - 2) × 12 = 36 TB
- After 3% overhead: 34.92 TB estimated usable
This is exactly the kind of scenario where a RAID storage calculator helps avoid surprises before deployment.
Final note
Use this tool for estimation and architecture planning. Real-world usable space can vary by controller behavior, filesystem, sector format, alignment, deduplication, compression, and vendor-specific implementation. For production systems, always validate using your exact hardware and software stack.