This paper presents a simple yet sophisticated analytical model to predict cumulative steam/oil ratio (CSOR) using material/energy balance and gravity-drainage theory in which CSOR is a function of average reservoir properties (porosity, permeability, heat capacity, and thermal conductivity) and time-dependent variables (injection temperature, rising chamber height, chamber oil saturation, and produced-fluid temperature) to mimic practical steam-assisted gravity-drainage (SAGD) processes. This model has been applied to predict CSORs of typical wells in the Japan Canada Oil Sands Limited (JACOS) Hangingstone SAGD project. History-match results show the reliability and accuracy of the model at different geological conditions. The analytical model has proved to be satisfactory in predicting CSOR by reasonably adjusting a limited number of reservoir parameters, such as formation permeability, effective net pay, and operational parameters.