The reliability of software relates to how often failures are produced by that software. Reliable software will produce a lower amount of failures. There are different measures of reliability:

- Mean Time To Failure (MTTF)
- Mean Time Between Failures (MTBF)
- Availability

MTBF and Availability are calculated using another measure: Mean Time To Repair (MTTR). Reliability is a good measure of software quality as it strives to consider how well it will perform under use. The way a system is used can have an affect on its reliability, so this needs to be taken into consideration as well.

Mean Time To Failure (MTTF)

MTTF is calculated by using the estimated "failure rate". The "failure rate" is the probability of failure.

Mean Time Between Failures (MTBF)

The MTBF is calculated using the equation: MTTF + MTTR.

Availability

The Availability is calculated using: MTTF / (MTTF + MTTR).

Faults and Failures

There are different types of faults and failures. Some faults in the code may have very little impact on the reliability of the system as they may have a very low probability of ever occurring.

Actual failures that occur due to faults in the code have differing levels of severity. Some failures may be insignificant whereas others may be critical, especially in the case of safety critical systems.

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