MTTR Calculation


MTTR definition

MTTR stands for Mean Time To Restoration [1], where restoration is achieved by either Repair or Replace.

An additional standard term is:

MRT: Mean Repair Time (not including undetected fault time and delays)

MDT: Mean Down Time – often MDT and MTTR are identical

Why calculate MTTR?

MTTR is a basic part of system availability and maintainability analysis.
System availability is affected by the downtime that each failure incurs.
Thus, a system that is harder to restore (due to difficulty in detection and isolation, hard to reach items, and/or delicate alignment) will incur higher downtime and reduce availability.

In addition to availability, MTTR also affects the expected maintenance man-hours and expected annual maintenance cost.


How to calculate MTTR?

If field data is available, MTTR is calculated as:

MTTR=Sum over fault times / total number of faults

If failure data is not available, MTTR can be predicted by assessing and summing the following time intervals [2]:

  • Detection, Isolation and malfunction verification
  • Preparation (e.g. warm up, power shut down)
  • Replace / Repair time
  • Alignment
  • Final Testing

Some standard task times for item replacement can be found in MIL HDBK 470A [3] and MIL HDBK 472 [2].
Example for “Nuts or Bolts”: remove time is 0.34 minutes, and replace time: 0.44 minutes.

Related Topics:
Spare Part waiting time
Testability and fault isolation

[1] IEC Electropedia (, IEC 60050
[2] MIL HDBK 472
[3] MIL HDBK 470A
[4] EN 61703 and EN 50126

BQR offers software and professional services for MTTR and Maintainability analyses.