How do I calculate MTBF from a failure log?

MTBF equals total operating time divided by the number of failures in that window. If a pump ran 2,000 hours and failed 8 times, MTBF is 250 hours. Count only operating time, not calendar time, and exclude planned downtime. The template does this automatically by summing the time-between-failure column and dividing by the failure count, so each new event updates the running average.

What is the difference between MTBF and MTTR?

MTBF measures reliability: the average operating time between two failures, so higher is better. MTTR measures maintainability: the average time to complete a repair once a failure occurs, so lower is better. MTBF answers how often something breaks; MTTR answers how fast you fix it. A machine with 300-hour MTBF and 2-hour MTTR is far better than one with 300-hour MTBF and 8-hour MTTR.

How does MTBF relate to availability?

Availability equals MTBF divided by the sum of MTBF and MTTR. With a 240-hour MTBF and a 4-hour MTTR, availability is 240 / 244, or 98.4 percent. This is inherent availability and ignores planned maintenance and supply delays. To raise it, either extend MTBF through better PM or shrink MTTR with spare parts and faster diagnostics. The template computes this ratio on every row.

What is a good MTBF value for factory equipment?

There is no universal target; MTBF is meaningful only against your own baseline or an OEM spec. A CNC spindle might run 4,000 hours between failures while a high-cycle pneumatic valve fails every 200 hours. What matters is the trend: a 20 to 30 percent MTBF gain after a reliability program is a strong result. Compare identical assets under identical duty to judge whether a specific machine underperforms.

How many failure events do I need for a reliable MTBF?

One or two failures give a noisy number. Aim for at least 8 to 10 events before trusting MTBF, and ideally a rolling window of 6 to 12 months. With few data points, one long or short interval swings the average heavily. The template's running average smooths this, but the monthly failure-count column tells you when your sample is still too small to draw conclusions.

Should planned downtime be included in MTBF calculations?

No. MTBF measures time between unplanned failures, so exclude planned maintenance, scheduled changeovers, and idle time from operating hours. Mixing them in inflates the denominator and hides the true failure rate. Log only failure events in the sheet and count actual run time between them. Keep planned downtime in a separate column if you want it for availability, but never let it distort the MTBF or MTTR averages.

Maintenance and Reliability

MTBF and MTTR Spreadsheet Template

Calculate mean time between failures and mean time to repair from a failure history log to track equipment reliability over time.

Overview

This template turns a raw failure log into two numbers that drive every maintenance decision: mean time between failures and mean time to repair. It is built for reliability engineers, maintenance planners, and plant managers who need to know whether an asset is actually improving. Tracking failures on paper or from memory hides trends. A spreadsheet forces you to record every event with real timestamps, so MTBF and MTTR reflect what happened instead of what you think happened.

You log each failure with a start time and a repair completion time. The sheet subtracts consecutive failure start times to get time between failures, and subtracts start from completion to get repair duration. MTBF is the average operating time between failures across the period; MTTR is the average repair duration. Availability is then MTBF divided by MTBF plus MTTR. The monthly summary counts failures per period so a rising failure count or falling MTBF shows up immediately in the trend.

In practice you baseline three to six months of history first, then compare MTBF and MTTR before and after a PM change or component upgrade. If MTBF climbs from 120 to 180 hours after a bearing redesign, you have proof the change worked. Feed the availability output straight into an OEE calculation. Pair it with the live MTBF Calculator to spot-check a single asset quickly, then use this sheet for the ongoing running average and trend.

What this template includes

Failure event log with dates and times
Failure duration per event
Time between failure calculations
MTBF and MTTR running averages
Failure count by period
Availability percentage derived from MTBF and MTTR
Trend summary by month

Suggested use case

Use this to baseline reliability before a maintenance improvement program, to track progress after a PM change, or to support an equipment replacement decision.

How to use it

Log each failure event with start time and repair completion time.
Calculate time between failures from the event log.
Review MTBF trend to see if reliability is improving.
Review MTTR trend to see if repair speed is improving.
Use availability to feed an OEE calculation.

Frequently Asked Questions

How do I calculate MTBF from a failure log?: MTBF equals total operating time divided by the number of failures in that window. If a pump ran 2,000 hours and failed 8 times, MTBF is 250 hours. Count only operating time, not calendar time, and exclude planned downtime. The template does this automatically by summing the time-between-failure column and dividing by the failure count, so each new event updates the running average.
What is the difference between MTBF and MTTR?: MTBF measures reliability: the average operating time between two failures, so higher is better. MTTR measures maintainability: the average time to complete a repair once a failure occurs, so lower is better. MTBF answers how often something breaks; MTTR answers how fast you fix it. A machine with 300-hour MTBF and 2-hour MTTR is far better than one with 300-hour MTBF and 8-hour MTTR.
How does MTBF relate to availability?: Availability equals MTBF divided by the sum of MTBF and MTTR. With a 240-hour MTBF and a 4-hour MTTR, availability is 240 / 244, or 98.4 percent. This is inherent availability and ignores planned maintenance and supply delays. To raise it, either extend MTBF through better PM or shrink MTTR with spare parts and faster diagnostics. The template computes this ratio on every row.
What is a good MTBF value for factory equipment?: There is no universal target; MTBF is meaningful only against your own baseline or an OEM spec. A CNC spindle might run 4,000 hours between failures while a high-cycle pneumatic valve fails every 200 hours. What matters is the trend: a 20 to 30 percent MTBF gain after a reliability program is a strong result. Compare identical assets under identical duty to judge whether a specific machine underperforms.
How many failure events do I need for a reliable MTBF?: One or two failures give a noisy number. Aim for at least 8 to 10 events before trusting MTBF, and ideally a rolling window of 6 to 12 months. With few data points, one long or short interval swings the average heavily. The template's running average smooths this, but the monthly failure-count column tells you when your sample is still too small to draw conclusions.
Should planned downtime be included in MTBF calculations?: No. MTBF measures time between unplanned failures, so exclude planned maintenance, scheduled changeovers, and idle time from operating hours. Mixing them in inflates the denominator and hides the true failure rate. Log only failure events in the sheet and count actual run time between them. Keep planned downtime in a separate column if you want it for availability, but never let it distort the MTBF or MTTR averages.