This paper discusses the development of a practical engineering tool to be used in predicting the per cent failures of mechanical components.In order to predict the per cent failures, it is necessary to convert life data of conventional S-N diagrams into strength data. The distribution function of these strength data is then determined. It was found in the present investigation that the three parameter Weibull distribution fits these data best.After finding the distribution function (Weibull) and its parameters for strength, the problem of distribution of stress is considered. On the basis of past experiences, the stress distribution is assumed to be normal. After values of the parameters of these interfering distributions of stress and strength have been determined, probabilities of failure of parts are calculated.An example is solved to demonstrate the application of the interference technique developed to predict per cent failures.