Space scientists have the task of managing long-life projects that last years, and eliminating failure of mission-critical equipment. Engineers predict the in-service reliability of components using data for mean time between failure, or MTBF. The MTBF for a piece of equipment that contains multiple components is dependent on the individual MTBFs, but the calculation can be complex. Using failures in time, or FIT, simplifies the math. FIT — the expected number of failures in one billion hours — is readily converted to MTBF in hours.

If you don’t want to do the math yourself, you can find calculator software online (see Resource 1).

Check your math thoroughly. It is easy to misread the number of zeros when dealing with numbers of this magnitude.

Note the value in FIT that you wish to convert to MTBF. Check that the value is given in failures per billion hours and write it down.

Divide 1,000,000,000 by the FIT value that you wrote down and note the result. For example, if the FIT value is 2,500, the result is 400,000 (see Reference 2, Sec.3.6, p.8-p.9).

Check your calculation. Record the result, which is the FIT value converted to MTBF, in hours.

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