Experiments were carried out on relatively large vertical propane sonic and subsonic exit velocity jet fires (up to approximately 10 m in length and 1.5 m in width). The main geometrical features of jet fires (flame shape, length and width) were determined by analyzing infrared images. From the observations of visible and infrared images, the flame boundary was defined as that corresponding to a temperature of 800 K. Results were compared with the shapes proposed in previous research projects. In the present study, data for sonic and subsonic exit velocity flames indicated that a cylindrical shape could accurately describe the shape of a vertical propane jet fire in still air. The length of such a cylindrical jet fire was the radiant flame length and the equivalent diameter was that corresponding to a volume equal to that surrounded by the aforementioned boundary. The ratio of flame length to diameter was found to be 7. Expressions are proposed to predict the values of jet flame length and width as a function of orifice exit diameter and Reynolds number. (C) 2010 Elsevier Ltd. All rights reserved.