HDD Failure Probability Calculator
Estimate annual hard drive failure probability from AFR and fleet size.
About HDD Failure Probability Calculator
Annualized Failure Rate (AFR, as published in studies like Backblaze's drive stats) gives a one-year failure probability per drive; compounding it over multiple years uses P(failure) = 1 − (1 − AFR)^years, assuming a constant hazard rate (a simplification — real drives follow more of a bathtub curve, with elevated infant-mortality and end-of-life failure rates). When you only have an MTTF/MTBF rating instead, AFR is derived from the exponential reliability model AFR = 1 − e^(−hours_per_year / MTTF). For a fleet of drives, the probability that at least one fails is much higher than any single drive's rate — this is the core justification for RAID redundancy at scale.
Formulas
Multi-year failure probability
P(failure over N years) = 1 − (1 − AFR)^N
- AFR
- annualized failure rate
- N
- years
AFR from MTTF
AFR = 1 − e^(−8760 / MTTF_hours)
- MTTF
- mean time to failure in hours
Worked example
With the default values below, HDD Failure Probability Calculator returns Per-Drive Failure Probability of 7.28%. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Input Basisone of Annualized Failure Rate (AFR), MTTF / MTBF (hours)
- Annualized Failure Rate (AFR)
- AFR (%)accepts 0–100
- 1.5
- MTTF / MTBF (hours)minimum 1
- 1,200,000
- Time Horizon (years)minimum 0.1
- 5
- Number of Drives in Fleetminimum 1
- 1
Results
- Per-Drive Failure Probability
- 7.28%
- Probability ≥1 Drive Fails in Fleet
- 7.28%
- Expected Failures in Fleet
- 0.07
- AFR Used in Calculation
- 1.500%
Step by step
- 1Values used
- 2Multi-year failure probability
- 3AFR from MTTF
- 4Per-Drive Failure Probability= 7.28
- 5Probability ≥1 Drive Fails in Fleet= 7.28
- 6Expected Failures in Fleet= 0.07
- 7AFR Used in Calculation= 1.500
Assumptions and limits
- The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 3 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 fleet failure probability so much higher than a single drive's AFR?
Because it's the probability that at least one of many independent drives fails, which compounds quickly: with a 1.5% AFR, a single drive has a 98.5% chance of surviving a year, but 100 drives together have only 0.985^100 ≈ 22% chance that all 100 survive — meaning a 78% chance at least one fails.
Is MTBF the same as expected lifespan?
No — MTBF/MTTF is a statistical rate derived from failures across a large population under the assumption of a constant (exponential) failure rate; it does not mean an individual drive is expected to run for that many hours. A 1.2-million-hour MTTF drive can still fail in year one, and virtually none are expected to run continuously for 137 years.
Does this account for drives aging (bathtub curve)?
No — this uses a constant hazard rate model, which is a simplification. Real drive failure rates typically follow a bathtub curve: higher in the first few months (infant mortality/manufacturing defects), lower during a stable middle period, then rising again as drives age past 3-5 years — AFR-based estimates are most accurate for that stable middle period.