RAID 5 Capacity Calculator
Estimate usable storage, parity overhead, and resilience for a single RAID 5 array.
What this RAID 5 calculator does
This calculator helps you quickly answer the most common planning question: how much usable space do I really get from RAID 5? It also gives practical context by estimating parity overhead, storage efficiency, and a rough rebuild duration.
RAID 5 is still popular in home labs, SMB NAS devices, and archival systems where you want better capacity efficiency than RAID 1 or RAID 10, while still surviving one drive failure.
How RAID 5 capacity is calculated
Core formula
For a RAID 5 group with equally sized drives:
- Usable capacity = (active drives − 1) × smallest drive size
- Parity overhead = capacity of one drive
- Fault tolerance = one failed active drive
If you configure hot spares, those spare disks are not counted as active capacity until a failure occurs. That means your immediate usable space goes down, but your recovery posture usually improves.
Why overhead matters
The number from a pure RAID formula is not the exact size your operating system will show. Real-world usable space is reduced by metadata, partitioning, filesystem structures, and occasionally block-size alignment choices. That is why this calculator includes an overhead input.
Example: quick RAID 5 planning scenario
Suppose you have 6 drives, each 8 TB, and no hot spare:
- Raw physical capacity: 48 TB
- RAID 5 usable before filesystem overhead: (6 − 1) × 8 = 40 TB
- Parity equivalent: 8 TB
- Efficiency: 40 / 48 = 83.3%
If you reserve 2% for filesystem overhead, your net figure is roughly 39.2 TB before accounting for snapshots or application-level reserves.
Strengths and tradeoffs of RAID 5
Why people choose it
- Good capacity efficiency compared with mirroring-heavy layouts
- One-drive fault tolerance without giving up half of your storage
- Widely supported in software RAID and NAS platforms
Where caution is needed
- Only protects against a single drive failure
- Long rebuild times on large disks increase exposure window
- Write performance can be lower due to parity calculations
- Not a backup solution by itself
RAID 5 vs RAID 6 vs RAID 10 (capacity mindset)
If your drives are very large (for example 12 TB, 16 TB, 20 TB+) and uptime matters, many teams move from RAID 5 to RAID 6 for dual-parity protection. RAID 10 often wins on write performance and rebuild simplicity, but gives up more usable capacity.
- RAID 5: best capacity efficiency of the three, single-failure tolerance
- RAID 6: lower efficiency, dual-failure tolerance
- RAID 10: speed and rebuild advantages, but less usable space
Best practices when sizing a RAID 5 array
1) Use identical drives when possible
Mixed sizes work, but the smallest drive effectively defines usable size per disk across the set.
2) Keep a spare strategy
A hot spare can reduce time-to-rebuild response and operational stress during incidents.
3) Monitor SMART and scrub regularly
Early warning and data scrubbing catch latent issues before rebuild pressure reveals them at the worst time.
4) Always maintain external backups
RAID protects availability, not against deletion, malware, corruption, controller mistakes, or site loss.
Frequently asked questions
How many drives are required for RAID 5?
Minimum is 3 active drives.
Can RAID 5 survive two drive failures?
No. RAID 5 is designed to survive one failed active disk. A second failure before rebuild completion usually causes array failure.
Is RAID 5 still worth using?
Yes, in the right context: moderate-size arrays, good monitoring, and strong backup discipline. For very large arrays or critical workloads, RAID 6 or alternative layouts may be safer.
Why does my NAS show less space than the calculator?
Vendors may report decimal TB while systems display binary TiB, and filesystem/metadata overhead reduces visible capacity.
Bottom line
A RAID 5 calculator is useful for early architecture decisions, budgeting, and storage forecasting. Use it to estimate usable capacity, but pair it with operational safeguards: backups, monitoring, and realistic rebuild planning. Capacity is important; recoverability is critical.