RAID 10 Capacity Calculator
Use this tool to estimate usable storage for a RAID 10 array with optional hot spares and filesystem overhead.
RAID 10 requires an even number of active drives. 4+ is common.
Hot spares improve resilience but do not add usable capacity.
How RAID 10 capacity works
RAID 10 (also written RAID 1+0) combines mirroring and striping. Drives are grouped into mirrored pairs, then data is striped across those pairs. You get excellent performance and good fault tolerance, but you sacrifice 50% of your active raw capacity to mirroring.
That tradeoff is why RAID 10 is popular in virtualized environments, database servers, and performance-sensitive workloads where rebuild speed and predictable latency matter more than maximizing usable terabytes.
Core formula
Usable RAID 10 Capacity = (Active Drives ÷ 2) × Smallest Drive Size
- Active Drives = Total drives - Hot spares
- Active drives must be an even number
- All drives are effectively limited by the smallest drive in the array
- Optional overhead (filesystem, metadata, reserved space) reduces final usable space further
Why your “real” space looks smaller
Even if your RAID controller reports one number, operating systems can report another because of decimal vs binary units and filesystem metadata.
- Vendor drive size: usually decimal TB (1 TB = 1,000,000,000,000 bytes)
- OS display: often binary TiB (1 TiB = 1,099,511,627,776 bytes)
- Filesystem overhead: journals, metadata, reserved blocks, snapshots, alignment, etc.
Example RAID 10 scenarios
| Setup | Raw Capacity | RAID 10 Usable (Before Overhead) | Usable with 2% Overhead |
|---|---|---|---|
| 4 × 2 TB | 8 TB | 4 TB | 3.92 TB |
| 6 × 4 TB | 24 TB | 12 TB | 11.76 TB |
| 8 × 10 TB | 80 TB | 40 TB | 39.2 TB |
| 12 × 18 TB (2 spares) | 216 TB | 90 TB (10 active) | 88.2 TB |
Planning tips before you buy drives
1) Keep active drive count even
Because RAID 10 mirrors in pairs, an odd number of active disks is invalid. If you need hot spares, make sure the remaining active count still stays even.
2) Match drive sizes whenever possible
Mixed drive sizes waste capacity because the smallest drive defines the effective size of every disk in that array segment.
3) Don’t forget growth headroom
For production systems, avoid running above 70–80% utilization. Performance and maintenance operations become harder when arrays are nearly full.
4) Capacity is not backup
RAID protects against drive failure, not accidental deletion, ransomware, corruption, or site-level disasters. Keep independent backups.
RAID 10 vs RAID 5 and RAID 6 (capacity perspective)
- RAID 10: ~50% efficiency, fast rebuilds, strong performance
- RAID 5: better capacity efficiency, but parity rebuilds can be stressful on large disks
- RAID 6: lower usable capacity than RAID 5, but stronger protection during failures/rebuilds
If your workload is IOPS-heavy and latency-sensitive, RAID 10 often wins despite lower capacity efficiency.
FAQ
What is the minimum number of drives for RAID 10?
Two drives can technically behave like a mirrored pair, but most practical RAID 10 deployments start at four drives.
How many drive failures can RAID 10 survive?
It can always survive at least one failed drive. It may survive multiple failures if no mirror pair loses both disks. Best-case simultaneous failures equal the number of mirror pairs (one per pair).
Can I add a hot spare?
Yes. A hot spare does not increase usable space, but it can automatically replace a failed drive to start rebuild sooner.