In engineering and telecommunication, the mean time between failures (MTBF) is the average time a system will operate without a failure. The MTBF is a commonly-quoted reliability statistic, and is usually expressed in hours (even intervals on the order of years are instead typically expressed in terms of thousands of hours).

It is generally calculated in one of several ways:

• An expected MTBF can be calculated on a statistical basis from the known failure rates of various components of the system.
• Through empirical testing of a single part, the length of a performance measurement period can be divided by the number of failures that have occurred during that period.
• Through empirical testing of a group of items, the total functioning life of the population of items can be divided by the total number of failures within the population during the measurement period.