RAID 5 Capacity Calculator
Estimate raw capacity, usable storage, parity overhead, and post-reserve capacity for a RAID 5 array.
What a RAID 5 calculator tells you
RAID 5 is a common storage level that balances speed, capacity, and fault tolerance. It stripes data across disks and stores distributed parity, which lets the array survive the failure of one drive. A RAID 5 disk calculator helps you answer practical questions before you buy hardware:
- How much total raw storage will I have?
- How much usable storage will remain after parity?
- What is my parity overhead percentage?
- How much capacity is left after filesystem or snapshot reserve?
RAID 5 capacity formula
For equal-sized drives, RAID 5 capacity is straightforward:
Usable Capacity = (Number of Disks − 1) × Disk Size
The “missing” one-disk worth of capacity is parity. That parity is distributed across all disks, but in total it equals the size of one drive.
Important assumptions
- All drives are the same size.
- No hot spare is included in this calculation.
- Manufacturer disk sizes are decimal (TB/GB), while OS tools may display binary units (TiB/GiB).
Quick example
Suppose you have 4 disks at 8 TB each:
- Raw: 4 × 8 TB = 32 TB
- Usable RAID 5: (4 − 1) × 8 TB = 24 TB
- Parity overhead: 8 TB (25% of raw)
- Fault tolerance: one drive failure
If you reserve 10% for snapshots or metadata, your practical usable space is approximately 21.6 TB.
Why RAID 5 is still popular
RAID 5 remains widely used for archive, backups, and moderate-performance workloads because it offers better efficiency than mirroring. You get much more capacity per disk than RAID 1, while still gaining single-disk protection.
That said, modern large-capacity drives can rebuild slowly, which increases risk during degraded operation. For heavy-duty production environments, many teams choose RAID 6 or RAID 10 instead.
Planning tips before deployment
1) Use matching drives
Mixing disk sizes usually wastes capacity, because arrays are often limited by the smallest disk in the set.
2) Keep free space available
Storage pools and filesystems perform better when not filled to 100%. Reserve a percentage for snapshots, growth, and operational safety.
3) Think beyond capacity
Capacity is only one dimension. Also evaluate:
- Rebuild time after failure
- IOPS and write penalty for parity RAID
- Backup and recovery strategy
- Monitoring, SMART alerts, and spare-drive policy
RAID is not backup
RAID protects against hardware failure, not accidental deletion, malware, corruption, or site loss. Keep separate backups with versioning and offsite copies. A simple 3-2-1 backup strategy is still essential.
Final takeaway
Use the calculator above to size your RAID 5 array quickly and accurately. It gives you raw, usable, and effective capacity estimates so you can make smarter hardware decisions before provisioning storage.