RAID Sizing Calculator

Every disk array gives up some installed capacity to protection, spare drives, and operating room. The drive labels may add up to hundreds of terabytes, yet the space that is sensible to provision can be much smaller. A useful RAID plan keeps those deductions visible instead of treating the raw total as usable storage.

Layout geometry determines the first deduction. Striping keeps every active drive available for data but provides no redundancy. Mirroring stores copies. Single-, dual-, and triple-parity layouts reserve one, two, or three drive-equivalents in each group. Grouped layouts repeat that geometry, so changing either drives per group or group count changes capacity and the number of installed drives.

Raw installed capacity

All active drives plus dedicated hot spares, multiplied by the selected drive size.

Usable before reserves

The data-drive equivalents left after mirror or parity overhead.

Planned usable capacity

Usable capacity after metadata allowance and the chosen fill target.

Capacity units must match the source of the number. TB and GB use decimal powers of 1000, as drive manufacturers commonly do. TiB and GiB use binary powers of 1024. Entering a 12 TB drive as 12 TiB overstates its byte capacity by about 10 percent before RAID overhead is even considered.

Equal drive sizes are another important boundary. A mixed-size group normally behaves as if every member were the size of the smallest usable member, but controller, filesystem, and expansion behavior vary. Capacity arithmetic also says nothing about rebuild duration, workload latency, controller limits, failure correlation, or the chance of another fault during recovery.

RAID improves availability after some disk failures; it is not a backup. Accidental deletion, corruption, malware, fire, theft, and failures beyond the layout's tolerance still require separate recovery copies and tested restore procedures.

How to Use This Tool:

Build the array from its repeated group geometry, then reserve the capacity you do not intend to fill.

1. Choose RAID layout, then enter Drives per group and Group count. Correct any minimum-width, even-drive, or minimum-group warning before reading capacity.
2. Enter one member's Drive size with the matching decimal or binary unit, then add any Dedicated hot spares.
3. Set the Planning fill target and Metadata/system overhead to the reserves you want deducted from parity-adjusted capacity.
4. Enter a Chassis slot limit when bay fit matters, then compare Planned usable, installed drive count, and deployment warnings.

Interpreting Results:

Planned usable is the provisionable capacity after layout protection, metadata allowance, and fill headroom. Raw installed is a procurement total, not a safe data target. The efficiency percentage compares planned usable bytes with every installed byte, including hot spares.

• A no redundancy warning means RAID 0 has no modeled fault tolerance.
• A wide single-parity group warning appears at eight or more drives per group for RAID 5, RAID 50, or RAID-Z1. It is a review cue, not a predicted failure probability.
• Limited operational headroom appears only when the fill target is greater than 90%.
• A chassis over-limit result means active drives plus dedicated spares exceed the entered slot count.

Technical Details:

The model treats every group as an equal-member set and every dedicated spare as global installed capacity that contributes no data capacity. Protection is expressed in whole drive-equivalents, which makes the capacity path auditable across mirrors, parity groups, and repeated groups.

Formula Core

Let W be drives per group, G the group count, S dedicated spares, C bytes per drive, and D data-drive equivalents per group. Active and installed drive counts are:

If M is metadata overhead as a percentage and F is the fill target as a percentage, planned usable bytes P are:

Efficiency includes spare drives in the denominator. Values retain full byte precision; the interface formats capacity and percentages for display.

Rule Core

Data-drive equivalents and accepted group geometry follow these exact rules:

RAID 50 and RAID 60 require at least two groups. RAID 10 may survive more than one failure when failed drives belong to different mirror pairs, but the model reports only the one-failure guarantee. Chassis checking is disabled when the slot limit is zero.

Limitations and Accuracy Notes:

The result is a deterministic capacity plan, not a reliability or performance forecast. It assumes equal usable member sizes and subtracts only the entered metadata percentage and fill target.

• Filesystem formatting, checksums, snapshots, compression, deduplication, record size, controller metadata, and reserved system space may change real usable capacity.
• Hot spares add installed bytes and bays but do not add data bytes.
• Failure tolerance does not estimate unrecoverable read errors, rebuild exposure, correlated failures, or operator mistakes.
• Confirm supported layouts, width limits, sector formats, and expansion rules with the actual controller or storage platform.

Worked Examples:

References:

• What is Mirroring?, Storage Networking Industry Association.
• RAIDZ, OpenZFS documentation.
• Supported Levels for Intel RAID Controllers, Intel, last reviewed September 3, 2024.
• Definitions of the SI units: The binary prefixes, NIST.