Maintenance and Reliability

MTBF and MTTR: What They Mean and How to Calculate Them

MTBF is total uptime divided by number of failures. MTTR is total repair time divided by number of repairs. Here is how to calculate both and use them to make better maintenance decisions.

MTBF, mean time between failures, equals total uptime divided by number of failures. If a machine ran 720 hours last quarter and failed 6 times, MTBF = 720 / 6 = 120 hours. MTTR, mean time to repair, equals total repair time divided by number of repairs. If those 6 repairs took 18 hours total, MTTR = 18 / 6 = 3 hours. MTBF measures reliability, while MTTR measures maintainability, and you need both to understand equipment performance.

Equipment Availability links the two with the formula Availability = MTBF / (MTBF + MTTR). With MTBF of 120 hours and MTTR of 3 hours, Availability = 120 / 123 = 97.6%. Uptime, failure count, and repair duration usually come from CMMS work orders, machine logs, and operator downtime sheets. Typical MTBF ranges vary by asset type: CNC machines may run 1000 to 3000 hours between failures, while high-speed packaging equipment may be 200 to 500 hours. MTTR targets also vary, but simple mechanical repairs should often be under 30 minutes if parts and procedures are ready.

A common mistake is mixing planned stops with failures in the MTBF numerator and denominator. Scheduled PM, lunches, and planned changeovers are not failures and should not drag MTBF down. Another mistake is measuring MTTR only from wrench time while ignoring wait time for diagnostics, spares, or approvals. Plants also average all failures together, which can hide chronic small stops behind a few long events. Track by failure mode so you do not miss the real drivers.

Use MTBF to set PM intervals and to see whether reliability work is paying off. A common planning rule is to place PM frequency at roughly 50% to 70% of MTBF, so an 800 hour pump might get a PM around 500 hours. Use MTTR to attack maintainability with spare parts, troubleshooting guides, and technician training. If MTBF is already strong but MTTR is high, your next dollar belongs in repair readiness, not in more preventive work. These metrics also help explain OEE Availability losses in a language maintenance and operations both understand.

MTTR is often the fastest win because it responds quickly to process discipline. Three common drivers are no spare parts on-site, no written repair procedure, and technician skill gaps. Each one can usually be improved in weeks, not months. Related measures such as downtime cost, spare parts turns, and repeat failure rate help you decide where to focus next. MTBF and MTTR are only useful if you review them by asset class and failure mode, then change the maintenance plan based on what the numbers show.

Published 2026-05-28.