To calculate pressure losses during transportation of gas-liquid fluids in flowlines, it is important to correctly estimate the effective density and viscosity of the mixture, which increases the accuracy of calculating the tangential stresses required to determine hydraulic pressure losses at the liquid-wall boundary. The determining parameter when calculating the effective values of the density and viscosity of the mixture is the coefficient liquid holdup in the cross section of the pipeline. The article analyzes the applicability of empirical methods for modeling two-phase gas-liquid currents when calculating liquid holdup in slug body of a intermittent (slug) flow in horizontal pipelines. A description is given of the mechanistic approach proposed by Zhang to calculate the liquid holdup in the slug body used in one of the most famous modern models of gas-liquid flow — TUFFP Unified model. The results of calculating the actual liquid holdup in the slug body using the TUFFP Unified model hydrodynamic model are presented, showing good consistency of the calculated data with experimental ones for vertically located pipelines, and unsatisfactory consistency for currents in horizontal pipes. The purpose of the article is to improve the methods of calculating the gas-liquid slug structure of the current using the TUFFP Unified model, by improving the accuracy of calculating the liquid holdup in the slug body at the horizontal location of the flowline. The proposal to use in the TUFFP Unified model a new approach to assessing the stable diameter of the gas bubble of the dispersed-bubble flow regime in the body of a liquid slug made it possible to increase the reliability of calculating the liquid holdup in the slug body with a horizontal location of the pipeline.