Bottomhole Pressure (BHP) determination is quite crucial in the success of the drilling operations, especially on tight-drilling window environment. The calculation becomes much more critical when drilling with near-balance bottomhole pressure on the unconventional drilling techniques like Managed Pressure Drilling (MPD). Although Pressure-While-Drilling (PWD) technology can provide measured values of Bottomhole Pressure, it is so expensive and needs circulation to convey readings. Also, in case of tool failure, it consumes a lot of time to trip out of hole in order to change the tool, especially on deep-well drilling. On this paper, Bingham, Power-Law, Herschel-Bulkley and API RP13D rheological models have been utilized to calculate the Annular Pressure Loss (APL) on a deep well with polymer Water-Base Mud (WBM). A comparative study between model-calculated and PWD-measured values of APL for 15 points along the wellbore was conducted on the calculation of Annular Pressure Loss. Power-Law model was found the most optimum rheological model for polymer Water-Base Mud, due to exhibiting no Yield Stress. Power-law model gave 35 psi average error which can be tolerated most of the time during drilling operations.