The true cost of unplanned downtime: how to calculate lost hours, repair and the payback of predictive maintenance

The item that shows up in the maintenance budget is usually parts and labour. But when a line stops, the real cost is the value of the product that could not be made while it was down. That is why two plants can have the same repair invoice while one stoppage is far more expensive. Splitting the calculation into three parts moves the conversation from "what does maintenance spend" to "what does downtime cost us".

1. Cost per failure

The formula is simple:

• cost per failure = downtime hours × loss per hour + repair

An example line: 8 hours of downtime on average, a production loss of 50,000 TRY per hour, and 150,000 TRY of repair per failure.

• Loss: 8 × 50,000 = 400,000 TRY
• Repair: 150,000 TRY
• Total per failure: 550,000 TRY

Repair is less than a quarter of the total (150,000 / 550,000 ≈ 27%). The rest is invisible loss.

2. Annual cost

Assume 6 unplanned stoppages a year:

• annual cost = 6 × 550,000 = 3,300,000 TRY
• annual downtime = 6 × 8 = 48 hours

These two numbers are the most useful pair for making a maintenance meeting concrete: how much money, how many hours.

3. How much does early warning save?

Not every failure can be prevented. Vibration, temperature or current data shows some failures days ahead; others come suddenly. The calculation needs two honest assumptions:

• avoidable share: the percentage of failures that early warning can catch (25% in the example)
• planned-intervention ratio: what a caught failure costs as a planned stop, as a percentage of the unplanned cost (30% in the example)

The formula:

• savings = number of stoppages × avoidable share × cost per failure × (1 − planned-intervention ratio)

In the example: 6 × 0.25 × 550,000 × 0.70 = 577,500 TRY per year. Annual downtime also falls from 48 hours to 36, a reduction of 12 hours (48 × 0.25).

4. Payback on the investment

If the system costs 1,200,000 TRY, payback is 1,200,000 / 577,500 × 12 ≈ 24.9 months, roughly two years. How much does changing the avoidable share move the result?

• 10% → annual savings 231,000 TRY
• 20% → 462,000 TRY
• 30% → 693,000 TRY
• 40% → 924,000 TRY

Savings are directly proportional to the avoidable share, so that is the assumption most worth debating. Classifying your past failure records by asking "did this failure show a warning sign beforehand" turns the number from a guess into a measurement.

Four common mistakes

• Counting the repair invoice as the downtime cost and leaving out the lost hours.
• Not stating whether the hourly loss is based on gross revenue or contribution margin. Raw material and energy that are not consumed during a stop reduce the loss; say which one you used.
• Assuming an avoidable share close to 100%. Realistic starting values are low and rise as data accumulates.
• Deciding on a single scenario. Looking at four values, like the sensitivity table above, ties the decision to a range rather than to one assumption.

Try it with your own numbers

The unplanned downtime cost tool runs in your browser; the values you enter are not sent to any server. The defaults in the example are only examples, so enter your own line's numbers. For a predictive maintenance pilot, get in touch.