In drilling operations, borehole breakout formations considered as one of the main wellbore stability issues that might be faced. Generally, the drilling operations are designed to avoid the development of borehole breakouts and induced fractures. However, in some cases drilling engineers may accept a slight collapsing in the well for the cost of dropping the applied mud. In these cases, the drilling operations can be designed with an allowable level of breakout width. The long axis of the developed borehole breakouts is always aligned in the direction of the minimum horizontal stress in the fields. Borehole breakout width is significantly affected by the existing in situ stresses. In addition, the existing rock strength will control the application of an allowable breakout formations in the drilling operations. Therefore, adopting an appropriate geomechanical analysis which mimics field conditions is essential in such critical and sensitive study. This thesis presents a new 3D geomechanical model to predict the allowable breakout width in vertical wells using Mogi-Coulomb failure criterion with linear elastic consecutive solution of Kirsch. The commonly known Mohr-Coulomb criterion is used to verify the effect of the intermediate principal stress in determining the breakout width. In this study, Excel program is utilized to develop the threedimensional numerical model of the breakout width as per the applied theories. The impact of the rock strength parameters and in situ stresses on borehole breakout is studied in different in situ stress regimes. It has been found that the friction angle, cohesion, and the minimum horizontal stress are the most critical parameters on the breakout width formation. Moreover, using the classical Mohr-Coulomb failure criterion that neglects the impact of the intermediate principal stress might result in misleading design for the drilling operations. The developed model for borehole breakout width analysis applied in three different field case studies. The studied fields show that using Mogi-Coulomb law to model the width of borehole breakouts gives results withing the applied filed conditions. Furthermore, adopting the current general practice of using MohrCoulomb failure criterion in the analysis will raise the uncertainty in borehole breakout width predictions.