Disk Life Estimate Calculator

Work out disk life estimate instantly with clear inputs, formula shown and shareable results.

About Disk Life Estimate Calculator

MTBF is a population statistic, not a lifespan: 1.4 million hours does not mean a drive lasts 160 years, it means about 0.6 percent of a population fails per year of continuous operation. Multiplying by fleet size gives the expected replacements to plan for, and holding roughly 1.5 times that as spares covers clustering.

Formula

Failure rate from MTBF

AFR = 1 - exp(-annual power-on hours / MTBF); expected failures = drives x (1 - exp(-total hours / MTBF))

Worked example

With the default values below, Disk Life Estimate Calculator returns Annual failure rate of 0.624%. Change any input above and every figure here is recalculated in your browser.

Inputs used

MTBF ratingminimum 1,000 h: 1,400,000 h
Power-on hours per dayaccepts 0.1–24 h: 24 h
Drives in the fleetminimum 1: 200
Years in serviceminimum 0.1 years: 5 years

Results

Annual failure rate: 0.624%
Expected failures over the service period: 6.2
Expected failures per year: 1.2
Spares to hold: 2

Step by step

1Values used
2Failure rate from MTBF
3Annual failure rate= 0.624
4Expected failures over the service period= 6.2
5Expected failures per year= 1.2
6Spares to hold= 2

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, 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

Do observed failure rates match MTBF ratings?

Often not. Large-scale field studies report annual failure rates of 2 to 5 percent against ratings implying under 1 percent, partly because MTBF excludes infant mortality and end-of-life wear.

What is the bathtub curve?

Failure rates are high initially from manufacturing defects, low and flat through mid-life, then rise as mechanical wear accumulates. A constant AFR only describes the flat middle.