This paper presents a methodology for calculation of a slug, two-phase flow HOLD-UP in a horizontal pipe. The advantage of this method is that the slug unit hold up can be calculated directly from the solutions of flow field equations with no need to use correlations. An experimental apparatus to measure air-water hold up was setup. The flow pattern and liquid holdup in horizontal and inclined pipes, from angles 5o to 40o, for air-water two-phase flow, are experimentally observed. The test section, with an inside diameter of 30 mm and 3 m in length, was made of plexy-glass to permit visual observations of the flow patterns. The proposed model was tested extensively against experimentally collected data. Furthermore, other data sources for slug flow in horizontal pipes, for air-water and air-oil systems, were also used for comparison. The presented methodology was compared against four recently developed models of a two phase, slug flow holdup in horizontal pipes. Not only does the presented model demonstrate good agreement, with less than 6.8% error, compared to the experimental data, but also has less error compared to other models. These results substantiate the general validity of the model.