The stochastic features of vapor bubble departure and lift-off diameters in forced convection boiling are considered in terms of probability density functions (pdfs). A mechanistic model is developed for the prediction of these functions. Their prediction requires a vapor bubble detachment model and pdfs of the mean wall superheat and mean liquid velocity. The agreement with horizontal flow boiling vapor bubble departure and lift-off data is very good. To the best of the authors’ knowledge, this is the first attempt at developing such a predictive capability. In order for the model to be generally applicable to flow boiling systems, the assumptions used in estimating the mean wall superheat and mean vapor velocity pdfs require further scrutiny.