RAID Capacity Calculator

Usable capacity, fault tolerance, and the rebuild risk nobody prints.

What you provide

Result

CHECK

RAID 5: 24.0 TB usable, survives 1 drive failure

Rebuilding this array means reading 24.0 TB without a single unrecoverable error. At the consumer drive specification that is roughly a 85% chance of hitting one, and with only 1 drive of redundancy an error during rebuild can mean losing the array. This is the arithmetic behind the advice to use RAID 6 rather than RAID 5 on large drives — a second parity drive costs capacity and removes this cliff.

Diagnostic telemetry
Usable capacity
24.0 TB
Raw capacity
32.0 TB
Capacity used for redundancy
25%
Drive failures survived
1
Rebuild readsEvery byte of the surviving drives has to be read to rebuild
24.0 TB
Chance of a read error during rebuildAt the consumer specification of one unrecoverable error per 10^14 bits
85%
Usable in TiBWhat the operating system will report, using binary units
21.8 TiB

What this cannot tell you

  • Calculated in your browser. Nothing you enter is uploaded or logged.
  • The rebuild risk is an order-of-magnitude estimate from the manufacturer's unrecoverable read error specification of one per 10^14 bits. Real drives usually beat their specification and controllers differ in how they handle a single bad sector, so treat it as an argument for double parity rather than a probability to bet on.
  • Usable capacity is the array arithmetic only. Filesystem overhead, formatting and the difference between TB and TiB all reduce what you can actually store, and the TiB figure is shown for that reason.
  • RAID is availability, not backup. It protects against a drive dying and does nothing about deletion, ransomware, a failed controller, or the building.
  • Cannot account for a specific controller's behaviour, hot spares, or nested and vendor-specific levels.

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