Spares Calculator Terms

Here is a list of terms used throughout the Spares Calculator package.

Units in Service

The number of operational units supported by the store.

MTBF (Mean Time Between Failure)

A measure of how reliable a hardware product or component is. The MTBF is equal to the average number of hours that will elapse before a component fails. For most components, the measure is typically in thousands or even tens of thousands of hours between failures. For example, a hard disk drive may have a mean time between failures of 300,000 hours. The MTBF figure can be developed as the result of intensive testing, based on actual product experience, or predicted by analyzing known factors. The manufacturer may provide it as an index of a product’s or component’s reliability and, in some cases, to give customers an idea of how much service to plan for.

NFF Ratio (No Fault Found Ratio)

A measure of how many components will be returned for repair that are still serviceable (i.e. not broken). The NFF Ratio depends on a number of factors such as the experience and training of users and support staff, the accessibility of the product, and the availability of tools, test equipment and facilities for testing the component prior to return. In specialist installations where the end user is supported by a team of highly experienced well trained and well equipped engineers the NFF ratio can be as low as 2%. In consumer applications or in installations where there is a high degree of user interface or where the component is easy to remove and return to the supplier this figure could be as much as 600%. A typical figure in most industrial applications is 50%.

Learn how to calculate the NFF Ratio

Repair Turn-Around-Time (TAT)

A measure of how long it takes for a spare part to be dispatched to the supplier, repaired and returned to the user’s store once it has been repaired.

Daily Operating Hours

A measure of the number of hours the equipment operates per day.

Total Fleet Operating Time

A measure of the total number of hours that the whole fleet of equipment operates during the Repair Turn-Around-Time period.

Mean Time Between Return

Derived by combining the MTBF and NFF Ratio and is equal to the average number of hours that will elapse before a component is returned for repair.


The probability that a spare part will not be available in the spares pool when demanded to carry out a repair. For example a stockholding of 10 spare parts might offer a 10% stock-out-risk. This would mean that 90% of the demands for spare parts would be satisfied during the Repair Turn Around Time and 10% of the demands would not.

Mean Time Between Stock-Out (MTBSO)

The average time that ought to elapse before a stock-out situation is experienced. For example a component with a 90-day repair turn-around- time and a 10% stock-out-risk should experience 1 stock-out situation in 10 repair periods. This would mean that there should be 1 stock-out situation in 900 days (i.e. 10 repair periods x 90 days). Therefore, the MTBSO would be 900 days or around 2.5 years.

Unit Cost

The cost of the product or component being analyzed.