RAID 10 Capacity Calculator
Estimate usable storage for a RAID 10 array, including optional hot spares and filesystem overhead.
What this RAID 10 calculator does
RAID 10 (also written RAID 1+0) combines mirroring and striping. You get better performance than a simple mirror, and stronger fault behavior than parity arrays during rebuilds. The trade-off is capacity: only about half of active disk space is usable for data.
This calculator helps you estimate:
- Raw capacity (all drives you bought)
- Usable RAID 10 capacity before filesystem overhead
- Estimated formatted capacity after overhead percentage
- Practical efficiency including optional hot spares
RAID 10 capacity formula
If all drives are equal size, RAID 10 usable capacity is:
Usable = ((Total Drives − Hot Spares) / 2) × Drive Size
Then apply your overhead estimate:
Formatted Usable = Usable × (1 − Overhead%)
Example: 8 drives × 4 TB, no spares: usable = (8 / 2) × 4 = 16 TB (before filesystem overhead).
Important planning rules
1) Active drives must be even
RAID 10 is built from mirrored pairs. After subtracting hot spares, the remaining active drives must be an even number.
2) Minimum is 4 active drives
Technically, RAID 10 starts at four active drives (two mirrored pairs striped together).
3) Mixed drive sizes waste space
Most RAID implementations size each mirror leg to the smallest drive in the set. If you mix sizes, extra capacity on larger drives may go unused.
How many drive failures can RAID 10 survive?
RAID 10 always survives at least one drive failure. It can survive multiple failures if they occur in different mirror pairs. But if both drives in the same pair fail, the array fails.
- Best case: up to one failed drive per pair
- Worst case: two failures in the same pair can cause data loss
- Reality: failure pattern matters, not just failure count
RAID 10 vs RAID 5/6 (quick view)
- RAID 10: Fast rebuilds, strong random I/O, ~50% usable efficiency.
- RAID 5: Better capacity efficiency, but riskier and slower rebuild behavior on large disks.
- RAID 6: Better failure tolerance than RAID 5, still parity-rebuild overhead.
Final note
RAID is not backup. Even with RAID 10, keep versioned backups and test restores. Use this calculator for capacity planning, then validate with your RAID controller or software RAID tool before deployment.