Redundant Array of Independent Disks (RAID) Calculator

A RAID plan fails early when raw drive labels are treated as the space users or applications can safely consume. Redundant Array of Independent Disks layouts combine multiple drives for performance, redundancy, or both, but every protection choice changes the capacity answer. Mirrors duplicate data. Parity reserves drive-equivalent space for recovery. Hot spares wait outside normal use. Filesystems, snapshots, fill limits, and operating reserve reduce the number again.

The useful capacity question is therefore staged. Installed raw capacity describes what is in the chassis. Usable RAID capacity describes what remains after the selected layout and smallest engaged disk size are applied. Planned usable capacity is more conservative because it also leaves room for overhead, growth, snapshots, and emergency recovery work.

RAID capacity and RAID safety are related, but they are not the same promise. RAID 6 can tolerate more disk failures than RAID 5 before data is lost, while RAID 10 depends on which mirror members fail. A layout may have an attractive capacity result and still spend many hours in a degraded state during rebuild, especially with large drives or slow rebuild throughput.

Backups remain separate from RAID. RAID can keep a system running through some disk failures, but it does not protect against deletion, ransomware, filesystem corruption, controller faults, site loss, or application mistakes.

How to Use This Tool:

Enter the disk inventory first, then compare the selected layout with the capacity ledger, failure envelope, matchup views, and topology strip.

1. Set Disk slots, Disk size, and the unit. Use decimal TB/GB for drive-label planning and binary TiB/GiB when your source capacity already comes from an operating system or storage report.
2. Turn on Define each disk manually when the shelf mixes capacities. Blank manual slots are treated as empty bays, and the inventory summary shows installed count, empty slots, smallest disk, and largest disk.
3. Choose RAID layout. For RAID 1 and RAID 10, set mirror width. For RAID 50 and RAID 60, set group count and disks per group.
   Nested layouts only engage complete groups. Extra active disks can become idle when group count and disks per group do not divide cleanly.
4. Enter Hot spares before reading capacity. In mixed-disk plans, the largest spare candidates are reserved first so standby space does not inflate usable capacity.
5. Open Advanced when you need filesystem overhead, target fill, safety reserve, per-disk system reserve, rebuild throughput, or annual disk failure rate to match your planning policy.
6. Fix any red Current layout is not buildable warning before trusting the selected layout.
   Common causes are too few active disks after spares, every installed disk assigned as a spare, or RAID 50/60 group geometry that requires more active disks than the plan has.
7. Confirm the topology strip, then read Capacity Ledger, Failure Envelope, RAID Matchup Chart, RAID Matchup Table, and Capacity Mix Chart. Save the assumptions with any result used for purchasing, migration, or workload commitments.

Interpreting Results:

Use Planned usable after reserve for commitments, not installed raw capacity. That figure already reflects the selected RAID layout, filesystem overhead, target fill, and safety reserve. Then check whether the failure envelope and rebuild posture match the risk you can accept.

A large capacity result can still be a poor plan when it has no redundancy, a wide single-parity group, many idle disks, or a rebuild estimate that keeps the array degraded for too long.

Technical Details:

RAID capacity math first decides which disks are installed, which are held as spares, and which active disks can participate in the selected geometry. Blank manual slots are ignored. RAID 50 and RAID 60 engage only complete RAID 5 or RAID 6 groups, so leftover active disks can be excluded from the usable layout.

Protected capacity is usually based on the smallest engaged disk. If a 16 TB disk joins a protected group sized by 12 TB members, the extra 4 TB does not increase that group's protected capacity. After equalization, the calculation subtracts per-disk system reserve, applies filesystem overhead, applies the RAID efficiency rule, then applies target fill and safety reserve to produce planned usable capacity.

Formula Core:

The core equations show how raw engaged capacity becomes planned usable capacity and how rebuild exposure is estimated from drive size, rebuild speed, and annual failure rate.

For eight 12 TB disks in RAID 6 with 5% filesystem overhead, the equalized raw capacity is 96 TB. After 5% overhead, 91.2 TB remains before RAID efficiency. RAID 6 with eight engaged disks uses six disk-equivalents for data, so protected capacity is 68.4 TB. An 80% target fill and 10% safety reserve leave about 49.25 TB as Planned usable after reserve.

The rebuild-window risk is a conservative comparison signal based on annual failure rate, estimated rebuild duration, exposed disk count, and the guaranteed tolerance threshold. It is not a full reliability model because it does not include drive age, correlated failures, latent sector errors, scrubbing, controller behavior, workload, or vendor-specific rebuild algorithms.

Limitations:

RAID capacity estimates help compare layouts, but they do not certify that a platform can build, perform, or recover the exact array. Confirm the hardware and operating environment before using a result for production planning.

• Check controller, NAS, filesystem, and operating-system support for the selected RAID level and group geometry.
• Confirm drive qualification, firmware behavior, sector format, and sustained rebuild throughput for the actual hardware.
• Keep backup and restore plans separate from RAID tolerance because RAID is not protection from deletion, ransomware, corruption, or site loss.
• Evaluate performance separately; this capacity result does not estimate write penalty, latency, cache behavior, or workload contention.

Advanced Tips:

• Use Define each disk manually for migrations and mixed shelves. The smallest engaged disk and idle disk count often explain surprising capacity losses.
• Compare TB and TiB only when the source units match. Drive labels are usually decimal, while many operating-system reports are binary.
• Keep Target fill at 80% to 90% for plans that need snapshots, growth, or rebuild workspace. Values above 90% trigger a headroom recommendation.
• Use a sustained Rebuild throughput value, not a burst benchmark. Large drives at low rebuild rates can push the status into Long rebuild window.
• Review RAID Matchup Table when the selected layout is not buildable. It can show a protected layout that fits the same installed disks.
• Check disk health before trusting spare and rebuild assumptions. A protected layout is weaker when multiple drives already show media errors or worn flash.

Worked Examples:

FAQ:

Glossary:

Engaged disk

A disk that participates in the selected RAID geometry after spares and idle active disks are excluded.

Equalized capacity

The engaged disk count multiplied by the smallest engaged disk capacity.

Hot spare

A standby disk held out of normal capacity so it can be used when a member fails.

Planned usable capacity

The conservative capacity left after RAID redundancy, overhead, target fill, and safety reserve.

Rebuild window

The estimated time needed to rebuild a failed member from the smallest engaged disk size and rebuild throughput.

Guaranteed tolerance

The failed-disk count the selected layout can survive without relying on failures landing in favorable groups.

References:

• Common RAID Disk Data Format (DDF), SNIA, March 27, 2009.
• Data Protection Best Practices, SNIA, January 27, 2025.
• Managing RAID, Red Hat Enterprise Linux 10 documentation.
• Re-Evaluating RAID-5 and RAID-6 for Slower Larger Drives, IBM.
• How to check disk health in Linux, Simplified Guide.