Companies use MTBF, MTTF and FIT data to show reliability for product or components and to determine how many they need to make. Manufacturers have different ways and conditions they use to calculate these values, this post will go into the general definition and how they come up with these numbers.

MTBF (Mean Time Between Failures) is a measure on how reliable hardware or a component is. It is defined as the time between failures of a given device, or system, and is normally measured in hours. This statistical value is meant to represent the average amount of time that will pass between random failures, for a large sample, over a long period of time for a given component. It also indicates system reliability that is calculated from known failure rates of various components in the system.

For Example:

If there are 1,000 products operating in the market; the number of units that will fail after 100 hours is estimated and the number of products that must be prepared is determined.

The calculation formula used in this case is 1,000 units × 100 hours = 100,000 hours/units.

The forecasted number of failures is 100,000 ÷ 50,000 = 2 units if the MTBF is 50,000 hours. So 2 units should be prepared to replace the failed units.

MTBF calculation values vary between manufacturers, as their calculation methods and calculation conditions are different. There are two kinds of calculation methods and various environmental conditions depending on how they are combined. This is why there are multiple forms of calculation results.

MTTF (Mean Time To Failure) indicates the time until a product becomes faulty after initial use.

The same calculation methods are used for both, but MTBF is referenced for products that can be repaired and MTTF is referenced to products that cannot be repaired, or the cost to repair is to great.

Normally MTBF is used to represent the reliability of power supply products, but also uses MTTF to represent products that are obviously difficult to repair such as compact on-board products.

FIT (Failure In Time) is a unit that represents failure rates and how many failures occur every 109 hours.

As an example, there is the occurrence of 10 failures for every 109 hours in the case of 10FIT.

FIT values can be calculated with the formulas below with the MTBF or MTTF shown in the reliability


The following posts have links to Texas Instrument and ON Semiconductor MTBF/MTTF/FIT search: Texas Instruments MTBF and FIT Reliability Data
On Semiconductor MTBF and FIT Reliability Data