RAID Storage Capacity Calculator
Calculate usable disk space and fault tolerance for server storage arrays.
RAID Capacity Analysis
How It Works
RAID combines multiple physical drives into a logical storage array using mirroring, striping, and parity to improve storage efficiency and fault tolerance.
This calculator estimates usable storage capacity and drive fault tolerance based on the selected RAID configuration.
Capacity assumptions and worked example
Redundant Array of Independent Disks (RAID) combines multiple physical drives into a single logical unit for data redundancy, performance improvement, or both. Because different RAID levels use varying methods of mirroring and parity, calculating the actual usable storage capacity can be complex. This tool computes the total usable space and fault tolerance based on your selected RAID configuration (1, 5, 6, or 10) and drive sizes.
| RAID Level | Capacity Logic |
|---|---|
| RAID 1 (Mirroring) | Total capacity equals the size of a single drive, as all data is duplicated exactly. |
| RAID 5 (Parity) | Total capacity is the sum of all drives minus one drive used for distributed parity data. |
For four 4 TB drives, RAID 5 gives (4 - 1) × 4 = 12 TB before filesystem overhead. This calculator treats RAID 1 as one mirror across all selected drives: four 4 TB drives provide 4 TB, not 8 TB. RAID 10 instead uses mirrored pairs and provides 8 TB in this example.
Assumptions: equal-sized drives, no hot spares, decimal TB, and no allowance for formatting, metadata or reserved capacity. RAID 6 uses (N - 2) × drive size; RAID 10 uses N/2 × drive size and requires an even drive count. RAID does not replace backups.
Reference: Red Hat RAID levels.
Video Surveillance (CCTV)
Estimate array capacity for an NVR. Retention time additionally depends on camera bitrate, recording schedule, compression and reserved space; this tool does not calculate it.
Access Control Databases
Design fault-tolerant storage for centralized access control servers, using RAID 10 where appropriate. Actual performance and availability depend on the controller, workload, rebuild state and the rest of the system.
Enterprise NAS Sizing
Determine the physical disk requirements when deploying a new Network Attached Storage appliance for corporate file sharing or virtualization backups.
Frequently Asked Questions
Why is the calculated storage less than advertised?
This calculator reports decimal TB. Some operating systems display binary TiB: 1 TB is about 0.9095 TiB. The bytes are unchanged by that unit conversion. RAID redundancy, filesystem metadata and reserved capacity are separate reductions.
Is RAID a replacement for backups?
No. RAID provides hardware fault tolerance (keeping systems online if a disk dies). It does not protect against accidental file deletion, ransomware, or catastrophic physical damage. Always maintain off-site backups.
Plan capacity beyond the headline number
For six 8 TB drives in RAID 6, enter six drives and 8 TB. The result is (6 − 2) × 8 = 32 TB before filesystem overhead, approximately 29.1 TiB. If one of those six physical drives is a dedicated hot spare, it is not an active array member: calculate with five active drives instead, giving 24 TB.
For mixed drive sizes, use the smallest member’s usable capacity for a conventional equal-stripe array and confirm the controller’s behavior. Vendor-specific hybrid arrays and distributed storage systems may follow different rules.
Understand fault tolerance
RAID 10 can survive failures in different mirror pairs, but losing both members of one pair can lose the array. Rebuilds increase exposure to additional failure; the count displayed here is a structural limit, not an availability guarantee. Keep separate tested backups and allow free capacity for growth and operational work.