Storage planning
RAID Usable Capacity Explained
Estimate usable RAID capacity for common levels, then account for parity, mirrors, decimal drive labels, spares and filesystem overhead.
RAID 0 uses all drive capacity, RAID 1 mirrors data, RAID 5 reserves the equivalent of one drive for parity and RAID 6 reserves two. With mixed drive sizes, common RAID layouts usually treat every member as the size of the smallest drive.
Capacity formulas at a glance
RAID 0 uses all drive capacity, RAID 1 mirrors data, RAID 5 reserves the equivalent of one drive for parity and RAID 6 reserves two. With mixed drive sizes, common RAID layouts usually treat every member as the size of the smallest drive.
Four 4 TB drive example
| Level | Raw | Approx. usable before formatting |
|---|---|---|
| RAID 0 | 16 TB | 16 TB |
| RAID 10 | 16 TB | 8 TB |
| RAID 5 | 16 TB | 12 TB |
| RAID 6 | 16 TB | 8 TB |
Why the operating system shows less
Drive labels use decimal TB, while software may report binary TiB or label binary values as TB. Filesystem metadata, snapshots, reserved space and a hot spare can reduce the amount available to store ordinary files.
RAID is not a backup
Redundancy can keep a system running after certain drive failures, but it does not protect against deletion, ransomware, controller errors, fire or theft. Maintain a separate tested backup and plan rebuild risk as well as headline capacity.
Failure tolerance and rebuild planning
Capacity is only one selection factor. RAID 5 commonly tolerates one member failure, RAID 6 two, and RAID 10 can survive some multiple failures depending on which mirrors fail. Rebuild risk grows with drive size.
Compare usable capacity with the dataset, growth, snapshots and free-space target. Do not count a separate backup as array capacity or store the only backup inside the same enclosure.
Quick answers
Frequently asked questions
How much space does RAID 5 lose?
The equivalent capacity of one member drive, before other overhead.
Can different drive sizes be combined?
Often, but conventional arrays may use only the smallest-drive capacity from each member.
Why is 12 TB shown as about 10.9 TiB?
The same bytes are being expressed with decimal and binary units.
Does RAID 0 protect against failure?
No. One member failure can make the entire striped array unavailable.
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