One of the anticipated objectives of laser welding is to produce deep penetration by forming a keyhole that entrains the energy over the depth without appreciable enlargement of weld width. The evolution of liquid/vapour interface over time considering the effect of interfacial phenomena like evaporation, homogeneous boiling, and multiple reflections makes the formation of keyhole geometry complex in nature. An analytical approach is developed explicitly to predict the keyhole geometry in the weldment during laser spot welding as well as linear welding. The model highly simplifies the heat transfer during the welding process by assuming an instantaneous linear heat source. The temperature field is then used to estimate the shape as well as the size of keyhole, which is arbitrary in shape in a discrete solution domain. This keyhole is considered as initial source of volumetric energy that is considered in a finite element-based Fourier heat conduction model. The distributed volumetric heat energy is adaptive in nature since it is mapped with the arbitrary volume of weld fusion zone at a time instant. The calculated results are validated with experimental results reported in independent literature.