In multi-phase computational fluid dynamic simulations, there exist many techniques to extract the interface cells and spatial locations. These techniques do not differentiate between free-surface locations and other phase-boundaries (e.g. bubbles or drops). Extraction of free-surface spatial locations is required in many multi-phase phenomena. However, air entrainment processes occur only at the free-surface. Flow and geometrical parameters at free-surface locations are required to estimate and predict the amount of air-entrained. In this work, we develop an algorithm to extract the free-surface spatial locations and corresponding flow parameters, estimated the local free-surface and the amount of gas holdup at each time step. This algorithm is used to extend the multi-phase solver in OpenFOAM CFD to estimate the amount of air entrained at each time step automatically. We compare the proposed method with a manually estimated volume fraction of air below the free-surface from the image generated in Para View. We show that the results of the proposed method match with that of the manually estimated volume fraction. © 2021 Begell House Inc.. All rights reserved.