Our discussion on reliability metrics and how to use these to inform your maintenance and reliability decision-making continues by discussing equipment downtime, specifically on the topic of Mean Downtime, or MDT, and how to use this metric to your advantage.

Key Takeaways

  • Mean Downtime is the average amount of time that an asset is required to be down to perform maintenance and repairs
  • Mean Downtime calculation formula = Total Downtime / # of Downtime Events
  • Downtime can be costly – a study found that downtime can cost up to $260k USD/hr.
  • Scheduled downtime is downtime that appears on a weekly maintenance schedule
  • Unscheduled downtime is the time an asset is down unexpectedly
  • Whereas MDT covers the entire time span that an asset, Mean Time to Repair is the average time it takes to repair an asset and strictly covers the time spent repairing the asse

Downtime is often associated with lost production, lost revenue, and sometimes even lost customers. 

The truth is that downtime is necessary – it is needed for maintenance, repairs, upgrades, and replacements. Where the distinction lies is how controllable the amount of downtime is and whether there is a system in place, such as a compatible maintenance program, to detect and measure how much time is spent on unplanned, prolonged, and costly downtime. 

This article is a continuation of our discussion on reliability metrics, specifically on the topic of Mean Downtime, or MDT, and how to use this metric to your advantage. Further, it shares insights on how to control the downtime of your assets. 

To begin the discussion, we must start at the beginning, which is by defining equipment downtime. 

What is equipment downtime?

Equipment Downtime is the timeframe that an asset is not in operation and results in a pause in production. For many manufacturers and maintenance teams, equipment downtime is the leading contributor to decreased production. It can be costly as well – a study suggests that downtime can cost up to $260k USD/hr.

Total Equipment Downtime - a comparison of scheduled and unscheduled downtime

Scheduled Downtime

Scheduled downtime is downtime that appears on a weekly maintenance schedule and includes the time spent from initial shut down to performing maintenance work on an asset. It ends when the asset is once again available and in operation. The frequency and criticality of scheduled downtime is typically the result of an organization’s maintenance philosophy.

Examples of scheduled downtime

Unscheduled Downtime

Unscheduled downtime is the time an asset availability is down unexpectedly and cannot perform its intended function. It has a direct impact on revenue as it includes more than a delay in production. A study by Efficient Plant found that unplanned downtime in manufacturing and heavy process industries “can impact 1-3% of revenue and potentially 30-40% of profits annually”.

It includes the delay time spent on repairs and modifications that are not on a maintenance schedule, including ordering parts when not available. In fact, close half of all unscheduled downtime can be traced to not having adequate spare parts.

Examples of unscheduled downtime incidents

  • Unexpected machine repairs (e.g., ruptured belt, broken component) 
  • Machine/Facilities Failure
  • Operational Issues
  • Process Failures
  • Emergency work

Equipment Downtime Analysis

Equipment downtime analysis is an important part of any reliability strategy. It involves assessing the percentage of time a piece of equipment is not operational due to factors such as maintenance, repair and/or replacement. By gathering data on downtime, organizations can pinpoint problems, identify solutions and optimize their maintenance and repair processes to reduce downtime and improve overall equipment reliability. Downtime analysis can also help organizations to identify areas for potential cost savings and improved productivity.

What is Mean Downtime (MDT)?

 Mean Downtime is the average amount of time that an asset is required to be down to perform maintenance and repairs. It starts from the initial equipment failure and encompasses the time spent diagnosing, lockout/tag outs, waiting for replacement parts, and repairing the asset. It ends when the asset is back online and completing its intended function. 

How to calculate MDT

The formula for MDT is the following:

Formula for mean downtime is total downtime divided by number of downtime events

MDT = Total Downtime / # of Downtime Events

Total Downtime = the amount of time an asset is not capable of running and is equal to the sum of Scheduled Downtime and Unscheduled Downtime.

Downtime Events = an event when an asset is down and is unable to perform its intended function.

Note: Mean Down Time is measured in hours or percentages.

What MDT calculations helps inform

  • Used as a KPI to track improvements in a site’s availability
  • Measures the effectiveness of an asset repair strategy, or its maintainability
  • Highlights inefficiencies in repair processes and can inform your critical spare parts strategy.
  • Tracks the progress of downtime reduction initiatives

The value of MDT is apparent – however, it is critical to also know how to keep this number under control when necessary.

Steps to Reduce Prolonged Equipment Downtime

The best way to reduce MDT is to enact preventive measures to avoid future downtime. Although it is easier said than done, there are several tried and proven methods for reducing downtime which simply require more strategic planning and scheduling.

  • Plan your downtime. Ideally scheduled downtime occurs when business is slow, such as after hours or times with slower demand. For some industries, this means planning maintenance activities during off-peak hours or holiday seasons

  • Build a maintenance strategy. A maintenance strategy that is both proactive and prescriptive will directly contribute to extending the life of critical assets and contribute to reduced risk of downtime. This “prescription” should be driven by your organization failure mode philosophy such as FMECA (Failure Mode, Effects and Critical Analysis), PM (Preventive Maintenance), and RCM (Reliability Centered Maintenance).

  • Document for efficiency. Speed up your repair and recovery time by ensuring there is a documented repair process with an efficient parts procurement and storage system in place. This is where a tool like a CMMS (Computerized Maintenance Management Systems) can help by keeping track of issues, work order history, location of replacement parts, and gathering the overall downtime.

  • Identify the root cause of issues. When an asset fails, particularly if it is a critical asset or experiences recurring failures, you should conduct a Root Cause Analysis (RCA) to figure out what may have been missed in the maintenance process that led to its failure. Once the root cause is found, preventive measures can be taken to reduce the risk of failure in the future.

Mean Downtime (MDT) vs Mean Time to Repair (MTTR)

Whereas MDT covers the entire time span that an asset, Mean Time to Repair is the average time it takes to repair an asset and strictly covers the time spent repairing the asset. In short, MDT provides a full picture of an assets downtime and can be used to identify areas where time is wasted and minimizes productivity.

The graphic below is a visual representation to help distinguish MDT and MTTR.


Mean Downtime is one of several maintenance metrics — MTTR, MTBF, and MTTF — which can be collectively used by maintenance teams to measure asset reliability. Reliability is what ensures asset performance and shapes the reputation of your organization, both inside and outside of the plant.

MaxGrip helps close the loop through our experienced engineering consulting services that pinpoint the exact areas for improvement that will decrease downtime and ensure reliable production. Our clients achieve this through our proven process called the Asset Improvement Program – in fact, it recently helped one of our clients achieve over $500k USD YoY savings.

The Asset Improvement Program benchmarks your site performance at an asset-level and provides a roadmap for tangible improvements. Learn how simple it is to get started on the road to improved asset performance by reading about the Asset Improvement Program.

🖥 Watch On-Demand: Foundational CMMS data

Get inspired

Ishikawa template

The fishbone diagram is a common tool to identify or prevent a particular event, download a template.

How Maintenance and Production can join forces to improve asset performance

Realizing a sustainable partnership between departments is important to improve production output, minimize TCO and to accelerate Digital Transformation.

Foundational CMMS Data

On June 14, 2022, 11:00 AM ET (17:00 CET), Terrence O’Hanlon, CEO Reliabilityweb will be hosting a 60 minute fireside chat to discuss the do’s and don’ts of Connected Frontline Worker solutions and successful adoption for an efficient asset management organization