RAID 5 Capacity Calculator
Use this tool to estimate raw capacity, usable storage, parity overhead, and RAID efficiency for a RAID 5 array.
How RAID 5 capacity is calculated
RAID 5 stripes data across all active drives and stores distributed parity information. The parity allows your array to survive the failure of one drive, but it also consumes the equivalent of one drive of capacity.
If all drives are the same size, this is easy to estimate. For example, 5 drives × 10 TB each gives 50 TB raw, and RAID 5 usable becomes 40 TB before filesystem overhead.
What this calculator includes
- Raw capacity of active drives.
- Parity cost (one drive worth of space).
- Usable capacity before and after your optional overhead percentage.
- RAID efficiency as a percentage.
- Single-drive fault tolerance, which is standard for RAID 5.
Important assumptions
1) Equal drive sizes
This calculator assumes all active disks have the same capacity. In real deployments with mixed sizes, RAID typically uses the smallest drive size across the array.
2) One-drive fault tolerance
RAID 5 can tolerate one drive failure. During rebuild, your array is in a vulnerable state. For larger arrays, many teams prefer RAID 6 for stronger fault tolerance.
3) Capacity shown is estimated
Actual usable storage can differ based on filesystem, block size, metadata, thin provisioning, and vendor implementation.
RAID 5 quick examples
- 3 × 4 TB → usable is about 8 TB before overhead.
- 4 × 8 TB → usable is about 24 TB before overhead.
- 6 × 12 TB → usable is about 60 TB before overhead.
RAID 5 vs RAID 6 vs RAID 10
If you are deciding on an array level, think about both capacity and resilience:
- RAID 5: good capacity efficiency, survives 1 drive failure.
- RAID 6: less usable capacity, survives 2 drive failures.
- RAID 10: excellent performance and resiliency profile, but lower capacity efficiency.
Best practices before deployment
- Keep a tested backup strategy (RAID is not backup).
- Use enterprise drives matched by model and firmware when possible.
- Monitor SMART, media errors, and rebuild times.
- Leave free capacity for performance and operational headroom.
- Validate controller and filesystem compatibility for your workload.
Final thoughts
A RAID 5 size calculator is great for planning, budgeting, and deciding how many disks you need. Use it as a practical estimate, then validate with your storage platform's exact behavior before purchase or deployment.