RAID Usable Capacity Calculator

This RAID usable capacity calculator estimates how much storage remains after a RAID layout, hot spares, and filesystem or reserved overhead are applied.

It is useful when drive labels show raw decimal capacity but storage systems report a smaller usable number. The result shows both decimal TB/GB and binary TiB/GiB so the difference is visible.

For pure unit conversion, use the data storage converter. This calculator adds RAID layout assumptions on top of the unit maths.

Six 8 TB drives give 48 TB of raw installed capacity. If one drive is assigned as a hot spare, five drives remain active for the RAID set.

In RAID 6, two active drives worth of capacity are reserved for dual-parity protection. That leaves three drives worth of capacity before filesystem overhead.

After a 3% filesystem reserve, the usable result is lower again. This is why a storage array's usable capacity can feel much smaller than the drive labels suggest.

RAID 0 and JBOD use all active drive capacity but provide no parity or mirror protection in this simplified model. RAID 1 mirrors drives and reports one drive of usable capacity.

RAID 5 reserves one active drive worth of capacity for parity, RAID 6 reserves two, and RAID 10 uses roughly half the active drives for mirrored pairs.

Actual implementations can vary by controller, filesystem, metadata layout, sector size, compression, snapshots, and reserved space.

Use file size calculator when the question is whether files fit on storage media, and data storage converter when the question is GB vs GiB or TB vs TiB.

Use this RAID calculator when drive count, drive size, RAID level, hot spares, and filesystem reserve affect the usable storage plan.

Before buying hardware or rebuilding a storage pool, confirm the result against your NAS, SAN, RAID controller, filesystem, backup design, and vendor documentation.

Start with installed drive count and labelled drive size, then subtract hot spares before applying the RAID layout. A spare is useful for recovery planning but it is not part of usable data capacity.

RAID levels reserve capacity differently. RAID 5 reserves one active drive worth for parity, RAID 6 reserves two, RAID 10 mirrors pairs, and RAID 1 reports one drive of usable mirrored capacity.

Add filesystem or reserved overhead when the storage system keeps metadata, snapshots, formatting reserve, or management space outside the data pool.

Six 8 TB drives give 48 TB raw installed capacity. With one hot spare, only five drives are active in the array.

RAID 6 reserves two active drives for parity, leaving three drives worth of usable capacity before filesystem reserve. A 3% overhead then reduces the final usable number.

The same bytes also look smaller as TiB than TB because decimal and binary units use different bases.

RAID 0 and JBOD use all active drive capacity in this simplified model, but they do not provide parity or mirror protection.

RAID 1 mirrors drives and reports one drive worth of usable capacity. RAID 5 reserves one active drive for parity. RAID 6 reserves two active drives.

RAID 10 uses mirrored pairs, so usable capacity is roughly half of active drives. With an odd active-drive count, this calculator rounds down to complete pairs.

Actual usable capacity can vary by controller, filesystem, vendor metadata, sector size, compression, deduplication, snapshots, reserved blocks, and pool layout.

RAID is not backup. It can improve availability or performance, but it does not protect against deletion, corruption, ransomware, theft, fire, or a failed restore process.

Before buying hardware or changing a storage pool, confirm the estimate against your NAS, SAN, RAID controller, filesystem documentation, and backup plan.

This page should target RAID usable capacity calculator, RAID storage calculator, RAID 5 capacity calculator, RAID 6 capacity calculator, RAID 10 capacity calculator, and hot spare capacity searches.

It estimates common RAID capacity from manual inputs. It does not design a storage architecture, benchmark performance, choose disks, calculate rebuild risk, validate backups, model ZFS vdev rules, or replace vendor sizing tools.

A common workflow is to paste or enter a real sample, review the output, then adjust one setting at a time. This makes it easier to see exactly what changed and avoid copying an incorrect result.

For developer and web-design tasks, test the result in the place it will actually be used. Encoded text, CSS values, parsed URLs, timestamps, and generated strings can behave differently depending on the target system.

Check formatting, character escaping, units, timezone assumptions, and browser support before using the output in production. Small formatting differences can break code, URLs, data files, or layouts.

Avoid pasting private secrets, passwords, API keys, or personal data into tools unless you are comfortable with where that data is processed. These calculators are designed for convenient local checks, not secure secret handling.

Developer utilities are most useful when they remove a tiny but annoying source of uncertainty. Instead of writing a scratch script, opening a terminal, or guessing a format, you can check the value quickly and move back to the main task.

That matters during debugging because small mistakes often hide in plain sight: a timezone offset, a copied user agent, an invalid UUID, a malformed URL, or a random token with the wrong length.

Before using the output in production, confirm the expected length, character set, timezone, casing, browser support, and validation rules. A value that looks right in isolation can still fail a strict API, database, CSS parser, or logging pipeline.

If the output will be shared with other people, label it clearly and include the assumptions used to create it. That turns a quick utility result into something another developer can trust and reproduce.