Introduction
MTTR and MTBF are two of the most important metrics in maintenance management. Understanding what they measure, how to calculate them, and how to use them is essential for any maintenance professional.
MTBF: Mean Time Between Failures
Definition
MTBF measures the average time between equipment failures. It indicates equipment reliability—higher MTBF means more reliable equipment.
Formula
MTBF = Total Operating Time / Number of Failures
Example
A pump operates 8,760 hours in a year and experiences 4 failures.
MTBF = 8,760 / 4 = 2,190 hours
On average, the pump runs 2,190 hours between failures.
Key Points
- Only count unplanned failures, not planned maintenance
- Use actual operating time, not calendar time
- Track separately for different failure modes if needed
- Higher MTBF = better reliability
What MTBF Tells You
- Equipment reliability trends
- Effectiveness of preventive maintenance
- Comparison between similar assets
- When to consider replacement
MTTR: Mean Time to Repair
Definition
MTTR measures the average time to repair equipment after a failure. It indicates maintenance efficiency—lower MTTR means faster repairs.
Formula
MTTR = Total Repair Time / Number of Repairs
Example
The same pump's 4 failures required 2, 4, 3, and 3 hours to repair.
MTTR = (2 + 4 + 3 + 3) / 4 = 3 hours
On average, repairs take 3 hours.
What to Include in Repair Time
Start: When failure is detected
End: When equipment returns to normal operation
Include:
- Diagnosis time
- Parts acquisition
- Actual repair work
- Testing and verification
Some organizations track MTTR variants:
- Mean Time to Respond: Time until technician arrives
- Mean Time to Restore: Total downtime including all delays
Key Points
- Consistent measurement criteria are essential
- Lower MTTR = faster repairs
- Don't confuse with Mean Time to Restore (includes waiting time)
Using MTBF and MTTR Together
Calculating Availability
These metrics combine to calculate equipment availability:
Availability = MTBF / (MTBF + MTTR)
Using our pump example:
Availability = 2,190 / (2,190 + 3) = 99.86%
The Reliability-Maintainability Balance
- High MTBF, Low MTTR: Ideal situation—reliable equipment that's quick to fix
- High MTBF, High MTTR: Reliable but slow to repair—ensure spare parts availability
- Low MTBF, Low MTTR: Unreliable but quick to fix—focus on root cause analysis
- Low MTBF, High MTTR: Worst case—prioritize for improvement or replacement
Improving MTBF
Strategies to increase time between failures:
Root Cause Analysis
Don't just fix failures—understand why they happen and prevent recurrence.
Preventive Maintenance Optimization
Schedule PM based on failure patterns, not arbitrary intervals.
Operating Procedure Improvements
Ensure operators follow best practices that extend equipment life.
Quality Parts
Cheap replacement parts often fail faster.
Design Improvements
Modify equipment to address recurring failure modes.
Improving MTTR
Strategies to reduce repair time:
Training
Skilled technicians diagnose and repair faster.
Parts Availability
Stock critical spares to eliminate waiting time.
Documentation
Provide repair procedures, troubleshooting guides, and equipment history.
Tool Availability
Ensure proper tools are accessible when needed.
Mobile CMMS
Give technicians instant access to information at the equipment.
Tracking in Your CMMS
A CMMS like FacilityLane automatically calculates MTBF and MTTR from work order data:
- Records failure times from work order creation
- Tracks repair duration from start to completion
- Calculates metrics by equipment, location, or category
- Shows trends over time
- Alerts when metrics cross thresholds
Common Mistakes
Inconsistent Definitions
Different people measuring differently makes data useless. Document standards clearly.
Including Planned Downtime
MTBF should only count unplanned failures. Planned maintenance doesn't indicate reliability.
Ignoring Context
A 100-hour MTBF might be excellent for one type of equipment and terrible for another.
Not Acting on Data
Metrics only matter if they drive decisions. Use them to prioritize improvements.
Conclusion
MTBF and MTTR are fundamental metrics that reveal equipment reliability and maintenance efficiency. Track them consistently, understand what they're telling you, and use them to drive continuous improvement.
FacilityLane automatically tracks these metrics and provides insights to help you improve equipment performance.