A wide range of physical and chemical processes of considerable industrial relevance rely on bubble flows. These include wastewater treatment, mineral and oil processing, fermentation or food treatment. A proper design of such processes requires the determination of hydrodynamic properties of single bubbles and of bubble swarms. This, in turn, involves the need for the development of reliable models, that are usually made of empirically derived correlations. There is a great number of experimental works dealing with determination of the hydrodynamic properties of gas bubbles rising in a stagnant liquid, but not many cover the case of co-current flow of dispersed gas and liquid. The aim of this study is to determine an empirical correlation to predict the drag coefficient of a single bubble during its formation in co-currently flowing liquid, as well as its equivalent diameter. A shadowgraphy technique is employed to determine experimentally the diameters of single gas bubbles formed in flowing in co-current liquid. The influence of the liquid viscosity on the forming bubble is evaluated. Based on the balance of forces affecting the bubble, the obtained experimental results are then used to determine correlations that permit to predict the drag coefficient and the bubble diameter. The derived models vary significantly from those available in the literature, but they show a much better agreement with the experimental data.