Mean Life, Service Life, and the Two-Thirds Rule of Thumb
- JD Solomon
- 1 day ago
- 4 min read
Asset managers and reliability engineers often talk past each other when the conversation turns to “life.” Service life, useful life, design life, and mean life carry different meanings depending on whether you sit in operations, finance, or engineering. The confusion is understandable. What matters is whether we can translate these concepts into decisions that improve reliability and reduce lifecycle cost. That’s where a simple rule of thumb—mean life is roughly two‑thirds of service life—can be surprisingly useful.
How Long Does That Pump Last?
“How long will that pump last?” asked the old salty operator as we all looked at the submersible pump sitting on the shop floor. Clearly, this was a challenge from the ranking member of the owner’s team to the ranking member of the consultant’s team as we began the next phase of a major project. There were 7 or 8 of our subordinates who immediately turned their eyes to me.
I knew it was a submersible pump and thought it was a Flygt. However, I could not tell whether it was a 4-inch or an 8-inch discharge. I couldn’t see the nameplate to get the horsepower. Technically, all of this matters; practically, it does not.
“We usually say around 20 years,” which was my default number for submersible pumps. “Some organizations say 30 years, but it really depends on the operating context and rebuild strategies.”
His head nodded. I waited for his next move.
But how did I know? Was it experience or luck?
It turns out that it was the two-thirds rule. The cut sheet for most submersible pumps will say their service life is something like 25, 30, or 25+ years. By experience, I have seen quite a few submersible pumps still in place after 30 years. Two-thirds of 30 is 20.
The critical bearings in a submersible pump running on a heavy-duty cycle (12 hours per day) are expected to last between 80,000 and 100,000 hours (18 to 22 years), so there is a strong reliability case for also stating 20 years.
The two-thirds rule works on just about every class of asset, even those that are not mechanical. Here’s why.
Service Life and Mean Life
Below is a quick summary of mean life and service life.
I have written several articles about asset life. If you want more details about the “big three,” see the following article.
For more on the many ways to calculate asset life, see also the following article.
Service Life Defined
Service life is a functional, operations-based concept. It reflects how long an asset performs its intended function at an acceptable level of risk and cost. It is shaped by operations, maintenance, environment, and economics.
Mean Life Defined
Mean life, on the other hand, is a statistical construct. It comes from fitting a probability distribution—usually Weibull—to failure data and calculating the average time to failure.
Never Perfect Alignment
Service Life and Mean Life rarely align perfectly, and they don’t need to. But when you’re planning budgets, setting replacement cycles, or communicating risk to decision makers, having a simple, defensible relationship between the two helps.
The Two-Thirds Rule of Thumb
The two‑thirds rule of thumb emerges from typical Weibull behavior. Mechanical assets often have shape parameters (β) between 2 and 4. When service life is defined at a high reliability threshold—say, the point where 90% or 95% of units are still surviving—the ratio of mean life to service life tends to fall in a narrow band.
It’s not a law of reliability engineering, and it’s not universal. But it is practical, repeatable, and close enough for early‑stage planning.
Comparing Shape Parameters
The table below illustrates this relationship. It compares mean life to the time at which 90% of units are still surviving for common Weibull shape parameters. The resulting ratios show why the two‑thirds heuristic persists.
A Practical Foundation
When failures are random (β = 1), mean life and service life are essentially the same. But as assets move into wear‑out behavior—where most mechanical equipment lives—the ratio stabilizes around 0.6 to 0.7. That’s the practical foundation for the two‑thirds rule.
Connecting for Decision Makers
Of course, no rule of thumb replaces real data. When we have a solid failure history, a Weibull analysis will always give us a better answer. But when we’re building a capital plan, briefing leadership, or estimating long‑term needs, the two‑thirds rule provides a clear, defensible starting point. The bridge between statistical reliability and functional asset management is exactly the kind of connection decision makers need.
Need help getting started? JD Solomon Inc. provides practical solutions to align asset useful life and strengthen your asset management and reliability program.
JD Solomon is the founder of JD Solomon, Inc., the creator of the FINESSE Fishbone Diagram®, and the co-creator of the SOAP criticality method©. He is the author of Communicating Reliability, Risk & Resiliency to Decision Makers: How to Get Your Boss’s Boss to Understand and Facilitating with FINESSE: A Guide to Successful Business Solutions.
