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RAID Failure Probability Calculator

Work out raid failure probability instantly with clear inputs, formula shown and shareable results.

About RAID Failure Probability Calculator

Data loss in parity RAID requires a second failure — or a third in RAID 6 — inside the rebuild window. The rebuild window is what makes the difference: a 37-hour rebuild is 0.42 percent of a year, so the conditional probability of a second failure is small but no longer negligible across eight disks. This model ignores unrecoverable read errors, which in practice dominate RAID 5 risk.

Formula

RAID failure model

P(first) = 1 - (1 - AFR)^disks; P(second during rebuild) = 1 - (1 - AFR x rebuild/8760)^(disks-1); RAID 5 loss = P1 x P2; RAID 6 = P1 x P2 x P3

Worked example

With the default values below, RAID Failure Probability Calculator returns Annual probability of data loss of 0.000002%. Change any input above and every figure here is recalculated in your browser.

Inputs used

Disks in the arrayaccepts 2–64
8
Annual failure rate per diskaccepts 0.01–50 %
1.5 %
Rebuild windowminimum 0.1 h
37 h
RAID levelone of RAID 5 - one parity disk, RAID 6 - two parity disks
RAID 6 - two parity disks

Results

Annual probability of data loss
0.000002%
Probability of any disk failing in a year
11.39%
Probability of a second failure during rebuild
0.0443%
Mean years between data-loss events
52,102,176.0 years

Step by step

  1. 1Values used
  2. 2RAID failure model
  3. 3Annual probability of data loss= 0.000002
  4. 4Probability of any disk failing in a year= 11.39
  5. 5Probability of a second failure during rebuild= 0.0443
  6. 6Mean years between data-loss events= 52,102,176.0 years

Assumptions and limits

  • The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax or fees or charges into account.
  • Figures are rounded for display to 6 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is the real RAID 5 risk higher than this?

Unrecoverable read errors. Reading tens of terabytes during rebuild at a URE rate of 10^-14 per bit makes encountering one quite likely, and that alone fails the rebuild.

Does RAID replace backup?

No. RAID protects against disk failure, not deletion, corruption, ransomware or fire. It is an availability mechanism, not a recovery one.

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