RAID Capacity Calculator

What Is the RAID Capacity Calculator?

The RAID Capacity Calculator plans storage for a NAS, DAS, server or workstation without confusing connection type with array capacity. Build the installed drive bays one by one, mark hot spares, and compare Single Disk, JBOD, RAID 0, RAID 1, RAID 5, RAID 6, RAID 10, RAID 50, RAID 60, RAIDZ1, RAIDZ2, RAIDZ3, Synology SHR and SHR-2. The result reconciles active raw capacity into usable data, parity or mirror equivalent, mixed-size capacity left unused, system reserve, filesystem or metadata estimate, physical free space, intentional free-space headroom and additional safe-to-use capacity. It displays correctly labeled decimal TB and binary TiB rather than silently mixing them. A separate expansion plan shows the theoretical before-and-after result of adding a member, assigning a hot spare or replacing a drive, then explains whether the selected platform allows that path and whether a rebuild, reshape, resilver, volume growth or full recreation may be required.

How It Works

Choose the storage system, implementation profile and RAID layout, then enter every installed drive in bay order. Mark standby drives as Hot Spare so they are excluded from usable capacity. Generic RAID 10 pairs adjacent active members, while RAID 50 and RAID 60 place contiguous active members into the selected parity groups. Open Capacity Assumptions to adjust per-drive system reserve, filesystem or metadata deduction, desired free-space headroom and space already used. Under Expansion Plan, add or replace a drive and compare current versus planned physical free space and safe-to-use capacity. Treat the mathematical gain and the in-place upgrade path as separate answers: verify the NAS model, controller, operating system and filesystem before changing a live array, and make a current backup first.

When to Use It

Use it when choosing drives for a new NAS or DAS, comparing RAID levels, checking why advertised drive totals differ from available space, planning a hot spare, mixing drive sizes, estimating free capacity after formatting and reserve, or working out whether one added or replaced disk will actually unlock more usable storage.

Frequently Asked Questions

Does NAS Or DAS Change RAID Capacity?

No. NAS and DAS describe how storage connects to clients. Capacity is determined by the drive sizes, active-member count, RAID or vendor layout, system reservations and filesystem. The enclosure and connection do affect performance and supported expansion workflows, so verify the exact model separately.

Why Does A Larger Replacement Sometimes Add No Space?

Classic RAID layouts are commonly limited by the smallest active member. Replacing one small drive with a larger drive can leave the same minimum size, so capacity does not increase until enough limiting members have been replaced, each rebuild has completed, and the array and filesystem are expanded.

Does A Hot Spare Increase Capacity Or Fault Tolerance?

No. A hot spare is excluded from usable capacity and does not add another simultaneous parity level. In a supported mirror, parity or protected SHR layout, a correctly sized spare can automatically replace a failed member and shorten degraded time. It cannot rebuild Single Disk, JBOD, RAID 0 or a one-drive SHR pool because those layouts have no redundant data to reconstruct.

Why Are Physical Free Space And Safe-To-Use Capacity Different?

Estimated Physical Space Free subtracts filesystem overhead and current usage from the array capacity. Additional Safe-To-Use Capacity also preserves the free-space headroom you selected for performance and future growth, so it is intentionally lower. Both figures are estimates rather than byte-exact filesystem readings.

Are Synology SHR Results Exact?

They are estimates using protected mixed-size capacity layers that match published Synology examples. SHR is system-managed, and the exact DSM version, NAS model, system partitions, filesystem and expansion rules can change the final result. The tool keeps the system-reserve assumption visible and editable.

Why Is RAIDZ Labeled Nominal Or Estimated?

RAIDZ space efficiency also depends on sector alignment, record size, dynamic stripe width, parity padding, metadata and ZFS reserved slop space. The calculator shows a planning estimate and separate deductions rather than claiming the simple parity formula is byte-exact formatted capacity.

What Is The Difference Between TB And TiB?

One decimal TB is 1,000,000,000,000 bytes, while one binary TiB is 1,099,511,627,776 bytes. A manufacturer-labeled 8 TB drive is about 7.28 TiB before RAID, formatting and system reservations. This calculator always labels both units explicitly.

Is RAID A Backup?

No. RAID can maintain availability after specified drive failures, but it does not protect against accidental deletion, corruption, malware, theft, enclosure failure, fire or a bad rebuild. Keep independent, tested backups.

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