How do you calculate MTBF?

Divide the total operating time by the number of failures in the same period. A machine that ran 1,000 hours with 5 failures has an MTBF of 1,000 ÷ 5 = 200 hours. Count only the time the machine was running, not repair time or time it sat idle with no orders.

How do you calculate MTTR?

Divide the total repair time by the number of repairs. If 5 repairs took 20 hours in total, MTTR is 20 ÷ 5 = 4 hours. Decide what the repair clock includes, such as waiting for a technician or parts, and use the same rule every period so the trend means something.

How do you calculate availability from MTBF and MTTR?

Availability = MTBF ÷ (MTBF + MTTR). With an MTBF of 200 hours and an MTTR of 4 hours, availability is 200 ÷ 204 = 98.04%. This figure counts only failures and repairs, not planned stops or changeovers, so it is usually higher than the availability factor in OEE.

There is no single good number: MTBF depends heavily on the type of equipment, how hard it runs and how failures are counted. The most useful comparison is the same machine's MTBF over time. A rising MTBF shows the equipment is becoming more reliable, and a falling one is an early sign of a developing problem.

What is the difference between MTBF and MTTF?

MTBF, mean time between failures, applies to repairable equipment that is fixed and put back into service. MTTF, mean time to failure, applies to parts that are replaced rather than repaired, such as bearings or bulbs, and measures how long they last on average. Using the wrong one gives a number that answers the wrong question.

What if there were no failures in the period?

Then MTBF cannot be calculated, because there is nothing to divide by. All you can say is that it is longer than the operating time. The calculator says so instead of showing a number; a longer period, or several similar machines together, gives a real figure.

Free tool

MTBF and MTTR calculator

Mean time between failures (MTBF) is the average time a repairable machine runs between breakdowns, and mean time to repair (MTTR) is the average time it takes to get it running again. MTBF is operating time divided by failures, MTTR is repair time divided by repairs, and together they give availability: MTBF ÷ (MTBF + MTTR).

MTBFequalsTotal operating time divided by Number of failures

MTTRequalsTotal repair time divided by Number of repairs

AvailabilityequalsMTBF divided by MTBF + MTTR

Use the same machine and the same period for every figure. Operating time is the time the machine was running; repair time is the time it took to restore it after each failure.

Operating time

Hours the equipment actually ran

Number of failures

Breakdowns in the same period

Total repair time

90 minutes = 1.5 hours

Number of repairs

Usually the same as failures

Pre-filled with the worked example. Use one machine or line and one period for every figure.

Mean time between failures

200 h

Average running time between breakdowns.

Mean time to repair

4 h

Average time to restore

Availability

98.04%

MTBF ÷ (MTBF + MTTR)

Failures per 1,000 hours

Failure rate, 1 ÷ MTBF × 1,000

How to calculate it

Four numbers from the maintenance log

Operating time is the hours the machine actually ran in the period. Leave out repair time, planned stops and time with no orders, or MTBF will look better than it is.

Failures are the breakdowns that stopped the machine. Agree what counts as one, for example any stop over five minutes that needed maintenance, and keep the rule the same from month to month.

Repair time and repairs give MTTR. The count of repairs is usually the same as failures; it differs when a failure needed more than one repair or a repair is still open at the end of the period.

MTBF and MTTR point at different fixes. A low MTBF calls for preventive maintenance and operator checks that catch problems early; a high MTTR calls for faster diagnosis, spare parts on hand and standard repair procedures. Both cut unplanned downtime.

Worked example

Illustrative numbers, not a benchmark

A machine that ran 1,000 hours in a period, broke down 5 times and took 20 hours in total to repair over those 5 repairs.

1MTBF = 1,000 h ÷ 5 failures = 200 h.
2MTTR = 20 h ÷ 5 repairs = 4 h.
3Availability = 200 ÷ (200 + 4) = 200 ÷ 204 = 98.04%.
4Failure rate = 5 ÷ 1,000 h = 5 failures per 1,000 hours.

MTBF 200 hours, MTTR 4 hours, availability 98.04%. The machine runs 200 hours on average between breakdowns and is back up in 4. Halving MTTR to 2 hours would lift availability to 200 ÷ 202 = 99.01%.

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