Redundant Array of Independent Disks (RAID) Calculator

Drive labels describe installed raw capacity, not the space that applications can safely consume. A Redundant Array of Independent Disks (RAID) uses striping, mirroring, parity, or a combination of them. Every protection choice spends part of the installed capacity, and ordinary operating reserves reduce the practical figure again.

Capacity planning is easier to audit as a sequence. Dedicated hot spares are removed first. The remaining drives are equalized to the smallest engaged member. The RAID layout then reserves mirror or parity capacity. Filesystem overhead, target-fill headroom, and an additional safety reserve are deducted in order. The final planned usable value is therefore deliberately smaller than both raw and layout-usable capacity.

Mixed drive sizes often create stranded space because a conventional set can use only the smallest member's capacity on every engaged drive. Nested layouts can also leave active drives idle when the requested group geometry does not consume them. Both losses matter before filesystem formatting begins.

RAID availability is not a backup. It does not provide an independent copy against deletion, ransomware, filesystem corruption, controller failure, site loss, or an application error. Capacity and tolerance figures should be paired with a tested backup and restore plan.

How to Use This Tool:

Describe the installed inventory before choosing how much of it the RAID layout and operating policy may expose.

1. Select uniform or mixed Drive inventory. For mixed drives, list every installed capacity in one consistent decimal or binary unit.
2. Choose the RAID layout, set parity-group geometry for RAID 50 or 60, and reserve any dedicated Hot spares.
3. Enter filesystem overhead, target maximum fill, and additional safety reserve. These deductions are sequential, so each percentage acts on the capacity remaining at that stage.
4. Add sustained rebuild throughput and annual drive failure rate only when you want the optional degraded-window estimate. Read Capacity ledger for the full waterfall and RAID fit for valid alternatives using the same drives.

Interpreting Results:

Planned usable is the shareable planning number after all selected deductions. The allocation view explains the difference from installed raw capacity as hot spares, idle drives, mixed-size loss, redundancy, overhead, target-fill headroom, and safety reserve.

• Tolerated failures reports the guaranteed layout-level count stated above; it is not the probability of data loss.
• Zero comparison capacity means that layout does not fit the active drive count or nested group settings.
• A rebuild estimate of zero means rebuild throughput was omitted or the layout has no protected rebuild path.
• The additional-failure percentage covers only the simplified Poisson overlap calculation during the rebuild window. It is not a complete reliability forecast.

Technical Details:

Every capacity is converted to bytes using the selected unit. Decimal units use powers of 1,000 and binary units use powers of 1,024. Drives are sorted largest first; when hot spares are requested, the largest listed drives are reserved before the active layout is evaluated.

Formula Core

For a fitting layout, engaged drives are capped at the smallest engaged capacity. The layout's data-member count then determines usable bytes before policy deductions.

The three planning deductions are applied sequentially, not summed into one percentage.

Here, O is filesystem and platform overhead, F is target fill, and R is the additional safety reserve. Bytes are formatted in the chosen display unit only after the calculation.

Rule Core

RAID 0 fits one or more active drives; RAID 1 needs at least two; RAID 5 needs three; RAID 6 needs four; and RAID 10 needs an even engaged count of at least four. RAID 50 requires at least two groups of three drives, while RAID 60 requires at least two groups of four. Nested layouts engage exactly the requested complete groups, leaving other active drives idle.

For mixed sizes, stranded capacity is the raw engaged total minus the equalized total. Redundancy is equalized capacity minus layout-usable capacity. These quantities remain separate so a large drive mismatch cannot be mistaken for parity cost.

Rebuild-risk estimate

When a positive rebuild rate and annual failure rate are supplied, the smallest engaged drive sets the rebuild duration. Throughput uses decimal MB/s.

The expected count of additional failures during that window is active peers multiplied by AFR and the fraction of a year spent rebuilding. The reported risk is the Poisson probability of at least the selected tolerance count. This deliberately narrow model assumes independent, constant-rate failures.

Limitations:

• Controller firmware may impose different member counts, grouping rules, spare behavior, metadata overhead, and displayed capacity.
• RAID 10 failure survival depends on which mirror members fail; the displayed tolerance is the guaranteed minimum.
• Rebuild speed may fall under foreground I/O, thermal limits, media errors, controller throttling, or distributed sparing.
• The model does not include unrecoverable read errors, correlated failures, enclosure faults, rebuild retries, snapshots, deduplication, compression, or thin provisioning.

Worked Examples:

References:

• Disk array capacities, IBM, September 9, 2020.
• A Case for Redundant Arrays of Inexpensive Disks, UC Berkeley EECS, 1987.
• How to check disk health in Linux, Simplified Guide.