Need to size a storage array quickly? This HDD RAID calculator helps estimate how much usable space you get after redundancy overhead. Enter drive count, per-drive capacity, RAID level, and optional hot spares to see a practical estimate of raw capacity, usable capacity, and parity/mirroring overhead.
How this HDD RAID calculator works
The tool uses standard RAID math with equal-size disks. If you mix drive sizes, most RAID controllers effectively treat every disk as if it were the size of the smallest drive in the set. In real deployments, firmware, filesystem formatting, and metadata reduce final formatted capacity slightly, so think of results as planning numbers.
Core assumptions
- All active drives are the same capacity.
- Hot spare drives are not part of active storage.
- Capacity results are decimal (TB) with a binary conversion (TiB) shown for convenience.
- Controller/filesystem overhead is not deducted.
RAID formulas used
RAID 0
Usable capacity = N × drive size
Fast and efficient, but zero fault tolerance. One failed drive can lose the whole array.
RAID 1
Usable capacity = drive size (for any mirror set of 2+ drives)
Maximum redundancy, minimum efficiency. In a full mirror, data can survive until only one copy remains.
RAID 5
Usable capacity = (N - 1) × drive size
Good balance of efficiency and protection. Can survive one drive failure.
RAID 6
Usable capacity = (N - 2) × drive size
Better protection than RAID 5 for large arrays. Can survive two simultaneous drive failures.
RAID 10
Usable capacity = (N / 2) × drive size (requires an even number of drives)
Great performance and strong resilience. Failure tolerance depends on which disks fail, but at least one drive can fail safely.
Choosing the right RAID level
- Need speed only: RAID 0 (temporary/scratch workloads only).
- Need simple redundancy: RAID 1 for small arrays.
- Need efficient bulk storage: RAID 5 for moderate risk profiles.
- Need safer large arrays: RAID 6 to reduce rebuild-risk exposure.
- Need performance + protection: RAID 10 for databases and VM workloads.
Practical sizing tips for HDD arrays
1) Plan around rebuild time
Large SATA disks can take many hours (or longer) to rebuild. During this period, the array is vulnerable. RAID 6 is often preferred for large-capacity HDD pools because it tolerates an additional failure during rebuild.
2) Keep spare strategy simple
Hot spares improve recovery response by reducing manual intervention. If uptime matters, include at least one spare in your design and budget.
3) Don’t ignore backups
RAID is not backup. It protects against drive failure, not accidental deletion, malware, corruption, or site events. Pair RAID with versioned backups and offsite copies.
4) Validate with your vendor docs
Different RAID controllers and NAS operating systems can reserve additional metadata space. Final formatted capacity can differ slightly from pure formula estimates.
Example calculation
If you have 8 drives × 12 TB in RAID 6 with 1 hot spare:
- Total physical raw = 8 × 12 = 96 TB
- Active drives = 8 - 1 = 7
- RAID 6 usable = (7 - 2) × 12 = 60 TB
- Array overhead (parity) = 84 - 60 = 24 TB
That gives you a resilient array with dual-parity protection and reserved spare capacity for quicker recovery.