Preventive Maintenance Interval Calculator

Work out preventive maintenance interval instantly with clear inputs, formula shown and shareable results.

About Preventive Maintenance Interval Calculator

For an exponentially distributed failure time, the interval giving a target reliability R is -MTBF x ln(R): 90% reliability needs an interval of about 0.105 x MTBF. The economic interval balances the cost of intervening against the expected cost of failure, and shrinks as failures become more expensive relative to planned work.

Formulas

Reliability-based interval

Interval = -MTBF x ln(target reliability)

Economic interval

Interval = MTBF x sqrt(2 x PM cost / failure cost)

Worked example

With the default values below, Preventive Maintenance Interval Calculator returns Interval For Target Reliability of 126.43. Change any input above and every figure here is recalculated in your browser.

Inputs used

Mean Time Between Failuresminimum 0.01 hours: 1,200 hours
Cost Of A Failureminimum 0: 250,000
Cost Of A Preventive Interventionminimum 0.01: 18,000
Target Reliability At Intervalaccepts 1–99.9 %: 90 %

Results

Interval For Target Reliability: 126.43
Economically Optimal Interval: 455.37
Interventions Per Year: 69.29
Failure To PM Cost Ratio: 13.89

Step by step

1Values used
2Reliability-based interval
3Economic interval
4Interval For Target Reliability= 126.43
5Economically Optimal Interval= 455.37
6Interventions Per Year= 69.29
7Failure To PM Cost Ratio= 13.89

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 compounding frequency into account.
Figures are rounded for display to 2 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

Does time-based PM always help?

Only for components with an increasing failure rate. For random failures, replacing a good part does not reduce risk, so condition monitoring is the better strategy.

Why is the reliability interval so much shorter than MTBF?

MTBF is the mean, and with exponential failures about 63% of items fail before it. Holding failure probability to 10% requires intervening at roughly a tenth of the MTBF.