RAID Capacity Calculator
Estimate raw, usable, and formatted storage for common RAID levels. Enter your drive details below and click calculate.
Assumptions: all drives are the same size, all active disks are in a single array, and RAID 50/60 are modeled as two equal groups.
How RAID capacity is actually calculated
A RAID array does not give you the simple sum of all drive sizes (except RAID 0 or JBOD). Some levels reserve capacity for mirroring or parity data, which protects against disk failure but reduces usable space. This calculator helps you estimate that tradeoff quickly.
At a high level, storage planning looks like this:
- Raw capacity = active drives × drive size
- Usable RAID capacity = raw capacity minus parity/mirror overhead
- Formatted capacity = usable capacity minus filesystem overhead
Quick RAID formula reference
- RAID 0: usable drives = N (fastest, no protection)
- RAID 1: usable drives = 1 (for full mirror set)
- RAID 5: usable drives = N - 1
- RAID 6: usable drives = N - 2
- RAID 10: usable drives = N / 2 (requires even count)
- RAID 50: modeled here as two RAID 5 groups, usable = N - 2
- RAID 60: modeled here as two RAID 6 groups, usable = N - 4
Minimum drive requirements and fault tolerance
Capacity is only one side of the equation. You also need to know what happens when drives fail:
- RAID 0: 1 failure destroys the array.
- RAID 1: can survive all but one drive in the mirror set.
- RAID 5: survives 1 failed drive.
- RAID 6: survives 2 failed drives.
- RAID 10: can survive multiple failures if they are in different mirror pairs.
- RAID 50/60: tolerance depends on how failures are distributed across groups.
For business data, RAID 6 or RAID 10 are often chosen because rebuild risk grows as drive sizes increase.
Why your operating system shows less space than advertised
Even after accounting for RAID math, OS-reported space may still look smaller than expected. Common reasons include:
- Manufacturers rate capacity in decimal bytes, while many systems display binary units.
- Filesystems reserve metadata and journal space.
- Controller-level formatting and partition alignment consume small amounts of space.
This is why the calculator includes a formatting overhead setting. A value around 3% to 8% is often realistic depending on filesystem and workload.
Planning tips before you buy drives
1) Start with required usable capacity
Decide how much actual usable space you need in production. Then work backward to determine drive count and RAID level.
2) Keep drive sizes matched
In mixed-size arrays, the smallest drive typically limits the usable size of every member disk, wasting capacity.
3) Include spares and growth headroom
If you expect growth, design for expansion from day one. Adding a hot spare can improve recovery speed, but it reduces active capacity.
4) RAID is not backup
RAID protects against disk failure, not accidental deletion, ransomware, corruption, or site-level disasters. Keep independent backups.
Example: 8 drives of 12 TB in RAID 6
If you configure 8 × 12 TB in RAID 6 with no spares:
- Raw = 96 TB
- RAID 6 usable = (8 - 2) × 12 = 72 TB
- After 5% overhead ≈ 68.4 TB
That is the kind of planning this calculator is designed to simplify.