RAID 50 Capacity Calculator
Enter your disk layout to estimate usable capacity, parity overhead, and fault-tolerance behavior for a RAID 50 array.
What this RAID 50 calculator does
RAID 50 combines striping (RAID 0) across multiple RAID 5 groups. This gives you better write performance and better rebuild characteristics than one very large RAID 5 set, while still keeping parity efficiency. The calculator above estimates:
- Raw capacity (all active drives before parity)
- Usable RAID capacity after RAID 5 parity overhead in each group
- Practical capacity after optional reserve percentage
- Failure behavior (how many simultaneous failures may be tolerated)
RAID 50 capacity formula
Core formula
For equally sized groups, RAID 50 usable capacity is:
Usable = (drives per group - 1) × number of groups × drive size
Each RAID 5 group “spends” one drive worth of capacity on parity. If you have 3 groups, you lose the equivalent of 3 drives to parity total.
Example
If you deploy 12 × 8 TB drives in 3 RAID 5 groups:
- Drives per group = 12 / 3 = 4
- Usable drives per group = 4 - 1 = 3
- Total usable drives = 3 × 3 = 9
- Usable capacity = 9 × 8 TB = 72 TB
Important RAID 50 rules
- Minimum drives: 6 (2 groups of 3 drives each).
- Minimum groups: 2 RAID 5 sets.
- Group size: each RAID 5 group needs at least 3 active drives.
- Balanced layout: active drives should divide evenly by number of groups for standard implementations.
How fault tolerance works in RAID 50
RAID 50 can survive one drive failure per RAID 5 group. So, in the best case, a 4-group RAID 50 could survive up to 4 simultaneous failures if each failed drive is in a different group.
However, if two drives fail inside the same RAID 5 group before rebuild completes, that group fails and the whole RAID 50 fails. Always treat “maximum survivable failures” as a layout-dependent best case, not a guarantee.
Planning tips for real-world deployments
1) Consider rebuild windows
Large-capacity drives can take a long time to rebuild, especially under load. Longer rebuild windows mean more risk exposure.
2) Use hot spares strategically
A hot spare can shorten time-to-rebuild and reduce operational risk. This calculator lets you model that impact on usable capacity instantly.
3) Keep drive sizes matched
RAID uses the smallest drive size in the array. If capacities differ, effective usable size may be lower than expected.
4) Don’t forget formatting overhead
Operating systems and filesystems report less than raw decimal capacity. The optional reserve field helps estimate practical capacity for snapshots, metadata, and growth headroom.
RAID 50 vs RAID 10 and RAID 60
- RAID 10: often better write latency and simpler rebuild behavior, but only ~50% capacity efficiency.
- RAID 50: better capacity efficiency than RAID 10, with good performance and moderate protection.
- RAID 60: double parity per group, higher fault tolerance, lower usable capacity than RAID 50.
Final takeaway
Use RAID 50 when you need a strong balance between speed, usable capacity, and resilience. For production planning, pair this calculator with your controller vendor’s recommendations, workload profile, and backup strategy. RAID is availability tooling—not a backup replacement.