Coal seams are converted to syngas by advanced thermo-chemical processes through Underground Coal Gasification (UCG) method. Inability to predict the shape and volume of the underground cavity is an important scientific gap in UCG method which is the main subject of this paper. For this purpose, firstly, a series of equations are introduced to predict the cavity growth dimensions over time. Subsequently, these equations are extended in numerical simulation of the Computational Fluid Dynamics (CFD), incorporating the commercial COMSOL software. According to the simulation, the amount of oxidant necessary to convert a certain amount of coal (in the heterogeneous phase) is calculated. The model results indicated that the shape and volume of cavity could be predicted at the onset of the gasification process. The numerical results agreed well with the field data. (C) 2018 Elsevier Ltd. All rights reserved.