You can enter MTBF and MTTR for 2 system components in the calculator above, from which the reliability of arbitrarily complex systems can be determined. MTBF values are usually provided by hardware manufacturers and MTTR will be determined by the processes you have in place for your system. For example, if one had a motherboard MTBF of 50000 hours, then adding a hard disk with an MTBF of 20000 hours will give a combined (or series) MTBF for the system of 14286 hours. Note a "failure" may not be unexpected, and could be planned, like in the case of a software upgrade for example.