Availability

A = MTBF / (MTBF + MTTR) - the fraction of calendar time the equipment is capable of operating.

MaintenanceReliability

MTBFhMTTRh

Availability: 99.701%

Number of 9s: 2.52 -

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Formula, assumptions, and limits

A = \frac{\text{MTBF}}{\text{MTBF} + \text{MTTR}}

Symbol	Meaning	Unit
$MTBF$	Mean Time Between Failures	h
$MTTR$	Mean Time To Repair	h
$A$	Availability

Source: IEC 60300 - Dependability management

In short

A = MTBF / (MTBF + MTTR) - the fraction of calendar time the equipment is capable of operating.

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