RAID 5 Capacity Calculator
Use this tool to estimate raw capacity, parity overhead, and usable storage for a RAID 5 array, including optional hot spare disks.
Note: RAID 5 tolerates one drive failure, but it is not a backup. Always keep separate backups of critical data.
How RAID 5 capacity is calculated
RAID 5 stripes data across multiple disks and stores distributed parity information so the array can survive a single disk failure. The simplest capacity formula is:
- Usable RAID 5 capacity = (active drives - 1) × drive size
- Parity overhead = 1 drive worth of capacity
If you configure a hot spare, that drive is not part of the active RAID set until a failure occurs. That means the spare helps with recovery speed, but it does not increase immediate usable storage.
Quick example
Suppose you install 6 drives at 10 TB each and reserve 5% for filesystem overhead:
- Total installed raw capacity: 60 TB
- Active RAID 5 drives: 6
- RAID 5 usable before reserve: (6 - 1) × 10 = 50 TB
- Estimated usable after 5% reserve: 47.5 TB
If one of those 6 drives is a hot spare, active drives become 5 and the RAID 5 usable capacity becomes 40 TB before reserve.
Why the “one-drive penalty” exists in RAID 5
Distributed parity
RAID 5 does not dedicate one fixed disk to parity. Instead, parity blocks are spread across all drives. Even though parity is distributed physically, the total parity overhead is still equal to one full drive of space.
Fault tolerance
RAID 5 can recover from exactly one failed drive in the active array. During rebuild, performance is typically reduced and risk is higher. A second disk failure before rebuild completion usually means data loss.
RAID 5 vs RAID 6 vs RAID 10
- RAID 5: 1-drive fault tolerance, good capacity efficiency, moderate write penalty.
- RAID 6: 2-drive fault tolerance, lower usable capacity, better safety for large arrays.
- RAID 10: High performance and resiliency, but only ~50% usable capacity in typical mirror+stripe setups.
If you are using very large modern disks, RAID 6 is often preferred over RAID 5 because rebuild windows can be long and stressful on remaining drives.
Best practices when planning a RAID 5 array
- Use identical drive models and capacities when possible.
- Include monitoring and alerting for SMART errors and degraded arrays.
- Keep at least one tested backup strategy (onsite + offsite is ideal).
- Consider a hot spare for faster recovery in business-critical systems.
- Account for real-world overhead: filesystem metadata, snapshots, and spare free space.
Frequently asked questions
Does RAID 5 improve performance?
It can improve read performance due to striping, but writes have parity overhead. Real performance depends on controller quality, workload type, cache behavior, and disk speed.
Can I mix different drive sizes?
You can, but the array usually treats all drives as the size of the smallest drive. That can leave extra space unused on larger disks.
Is RAID 5 a backup?
No. RAID protects against certain hardware failures, not accidental deletion, ransomware, corruption, or catastrophic events. Backups are still mandatory.