Slip ratio can be readily measured in an adiabatic two-phase flow experiment. However, in the flow boiling experiments, as the phase velocities keep varying, the estimation of slip ratio becomes difficult. In the absence of reliable methodology, the slip ratio for boiling flows is either taken simply as unity or deduced from void fraction experiments. It is noticed in literature that the experimental results reveal the effects of pressure and mass flux on void fraction and hence slip ratio. In the present work, numerical simulations to estimate slip ratio are carried out based on a well established wall boiling model. The computationally obtained void fraction values for subcooled boiling conditions in the vertical tube are verified with the available experimental data. The effect of operating variables, such as the tube diameter, the inlet mass flux and the degree of subcooling, on the variation of slip ratio is presented. The results show that the values of slip ratio vary along the length of the tube. A new correlation is proposed relating slip ratio with the thermodynamic quality, implicitly taking into account the effect of length and the operating conditions. © 2020 Elsevier Masson SAS