Array math

RAID Capacity Calculator

Calculate generic raw, usable, and protection-overhead capacity for a known same-size layout.

What this calculator assumes

All drives have the same decimal-TB capacity. A mirror uses one drive’s capacity; single parity uses one drive’s capacity; dual parity uses two. The operating system may show a smaller binary TiB value.

What it does not decide

Capacity math does not select the right protection policy. Use the RAID Protection Decision Tool and retain independent backup copies.

Decision guide

Use RAID Capacity Calculator with confidence

This guide explains the RAID arithmetic decision behind RAID Capacity Calculator, the evidence worth collecting for drive count and layout, and the boundaries of its raw-to-usable comparison. Work through RAID Capacity Calculator after a first run, then revise protection layout until the output reflects realistic conditions rather than convenience.

What this tool helps you decide

Calculate generic same-size RAID usable capacity, raw capacity, protection overhead, binary TiB display, and drive-failure tolerance. The RAID Capacity Calculator turns that RAID arithmetic question into a vendor-neutral raw-to-usable comparison, not a product endorsement. Use RAID Capacity Calculator before requesting quotes, because protection layout can change the sensible drive count and layout choice. Keep that scope visible whenever the raw-to-usable comparison is compared with alternatives.

Before you start

Select a generic mirror, single-parity, or dual-parity layout and enter equal drive count and decimal TB per drive. Confirm the layout’s minimum drive count before interpreting capacity. For RAID Capacity Calculator, write the RAID arithmetic assumption beside every saved result. That raw-to-usable comparison record lets another person reproduce the drive count and layout scenario, challenge protection layout, and rerun RAID Capacity Calculator after conditions change. Test one conservative RAID arithmetic case and one demanding case, then compare their raw-to-usable comparison range.

How the calculation or recommendation works

Raw capacity is drive count multiplied by drive size. Usable TB subtracts one drive-equivalent for single parity, two for dual parity, or half the drives for a mirror; displayed TiB converts decimal bytes to binary units. The RAID Capacity Calculator keeps RAID arithmetic arithmetic transparent, allowing the raw-to-usable comparison to be checked without a vendor account. In RAID Capacity Calculator, follow every drive count and layout unit shown by RAID Capacity Calculator. Decimal TB and TiB answer capacity questions. Keep every RAID Capacity Calculator input in its displayed unit; preserve those RAID Capacity Calculator labels when sharing or checking the result. Round the raw-to-usable comparison upward only after reviewing the unrounded RAID Capacity Calculator result.

How to interpret the results

Separate raw installed, protection overhead, usable decimal TB, and displayed TiB. Failure tolerance describes availability after limited drive failure, while the note reminds you that array redundancy is never an independent backup. Read the RAID Capacity Calculator summary with its RAID arithmetic metrics. Use the RAID Capacity Calculator table and caution note to examine the complete drive count and layout tradeoff. Save protection layout inputs with the result, compare scenarios under the same RAID arithmetic definition, and investigate conflicting raw-to-usable comparison output before acting.

Worked example

Four 12 TB drives provide 48 TB raw. In the default single-parity model, one drive-equivalent is protection overhead, leaving 36 TB usable, which displays as roughly 32.7 TiB before filesystem reserves. This RAID Capacity Calculator example demonstrates the RAID arithmetic path, not a universal default. Change protection layout one input at a time, observe which drive count and layout output moves, and note whether the raw-to-usable comparison remains stable. Sensitivity around RAID arithmetic is more useful than copying another person’s final number.

Assumptions and limitations

The result assumes same-size drives and simplified layouts. It excludes metadata, filesystem reserve, vendor hybrid schemes, hot spares, snapshots, thin provisioning, rebuild risk, and restrictions on adding or replacing drives. The RAID Capacity Calculator is a planning aid for drive count and layout in RAID Capacity Calculator. Treat the RAID arithmetic assumptions as boundaries: the raw-to-usable comparison supports RAID arithmetic planning only. It does not certify an implementation of RAID Capacity Calculator, guarantee recovery, or provide financial advice. Recheck protection layout, documentation relevant to RAID Capacity Calculator, and operating requirements before purchasing for RAID Capacity Calculator, migrating data, or approving a policy.

Next steps

Compare at least two protection layouts, confirm the exact NAS reports expected usable capacity, document tolerated failures, and calculate backup capacity independently of the RAID result. Continue from RAID Capacity Calculator to the Storage and RAID Capacity Formulas, then carry the RAID Capacity Calculator finding into another nas configuration tool for the next RAID Capacity Calculator drive count and layout decision. Keep the RAID Capacity Calculator raw-to-usable comparison with its assumptions. Record the decision date for RAID Capacity Calculator; assign a reviewer and a dated measurement checkpoint.