This paper evaluates a more accurate and relatively new shear rate equation based on the phenomenological model for the high yield stress drilling fluids to estimate frictional pressure drop in laminar and turbulent flow regimes under surface and downhole conditions. Fann viscometer readings are used to determine shear rates using the above phenomenological model that differ from the conventional shear rates, i.e., $\dot{\gamma }\left(s^{-\mathit{1}}\right)=\mathit{1.7}N$, where N = Fann viscometer rotor rpm. The new shear rates are used to predict model parameters of Bingham plastic (BP), power-law (PL), Herschel–Bulkley (HB), and Robertson-Stiff (RS) models, and corresponding frictional pressure drops. The efficacy of the above rheology models with improved model parameters is tested by validating the literature-reported experimental pressure drop data of low to high yield stress water-based drilling fluids. The validation results are consistent and accurately predict frictional pressure drops using HB and RS models with the lowest MSE-values. The above methodology is applied to predict frictional pressure drops in a pipe for ten test high-pressure, high-temperature (HPHT) drilling fluids of varying base fluids at the surface and HPHT conditions in laminar and turbulent flow regimes. For this, isothermal compressibility and thermal decomposition temperature of drilling fluids are measured to forecast the temperature- and pressure-dependent densities and shear viscosities. The proposed density model is experimentally validated and found to predict density at downhole conditions adequately with mean absolute- and standard-deviation, i.e., 1.67 and 1.36%, respectively. Temperature- and pressure-dependent viscosity and density have been used to predict pressure drop under downhole conditions using HB and RS models with improved model parameters from the phenomenological model. The HB model is found to be more accurate than the RS model for predicting pressure drop. The proposed frictional pressure drop estimation method in the present study is more general and accurate under the surface and downhole conditions than the conventional practice.