Abstract This paper focuses on developing a boil off gas model that can be used to predict values of boil off gas produced onboard a Moss spherical liquefied Natural Gas (LNG) Vessel. The Fourier law of heat transfer and Newton's law of cooling mathematical models were used to derive equations and then estimate the total heat transfer rate from the containment system under consideration. While maintaining the internal tank pressure, LNG composition and varying the ambient temperature and voyage length; using the concept of mean daily average temperature (MDAT), the boil off gas rate was estimated, hence a Moss Spherical LNG vessel boil off gas model was developed. Actual Boil off gas values were compared with values obtained by using the model developed in this study. The robustness of the equations derived allows the user to be able to develop various Moss Spherical boil off gas simulators for tanks with different types of materials. The statistical error analysis showed an Average Absolute Error of 5.63% and Standard Deviation of 7.3 when the results from this study were compared with actual boil off gas values obtained from a Moss Spherical LNG vessel.