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The general formula for availability is:

Availability = [MTBF/(MTBF+MTTR)] x 100

Availability is a function of the total service time, the mean time between failure (MTBF), and the mean time to repair (MTTR).


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  1. Andy S.

    Can you use this general formula also for hardware? Thanks on advance

  2. tarig A.


    please advice regarding the availability of the whole system ; i think the above availability is for a one service/link/node, so in case we have number of nodes occupied by number of links each link occupied by number of service how can i calculate the system availability.

    thanks for your swift feed back.

    Tarig Alamin

  3. Sean M.

    There are two more formulas for Service Availability, this one here relates only to a single instance of a productive unit.

    In Serial Mode:

    that is one Productive Unit (PU) after another PU the formula is:

    Service Availability = PU1 availability * PU2 availability (* PUn availability and so on)

    so 100% * 98% = 98%
    and 98% * 98% = 96.04% availability

    In Parallel mode:

    where PU1 and PU2 are linked such that one takes over should the other fail the PU Total availability needs to be calculated first:

    1 – (1-PU1 availability(original availabilty measure for single PU)))*(1-PU2 availabilty))

    so (PU1 and 2 are parallel such that PU2 takes over if PU1 fails) 1-((1-98%(PU1))*(1-98%(PU2)))=99.96%

    it is possible now to combine the serial and parallel availability calculations to create very complex Service Availabilities.

    I always draw out a diagram of how to work this out, makes life a lot easier.


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