This paper investigated the flame projection distance (defined as the distance from the nozzle exit to the furthest point of the flame in the horizontal direction) of horizontally oriented buoyant turbulent rectangular jet fires. The experiments as well as the correlations reported previously were limited for axi-symmetrical fire sources that are not applicable for non-axi-symmetrical sources. In this work, experiments were conducted using horizontally oriented rectangular nozzles with aspect ratio, n (nozzle length to nozzle width: n= L/W), varied from 1:1 to 71:1 covering the axi-symmetrical, rectangular and linear sources, employing propane as fuel. Results showed that the flame projection distance increased with the heat release rate growth. At the same time, the flame projection distance decreased with the increase of the aspect ratio n for a given nozzle exit area. A non-dimensional function was then derived for the projection distance, in which a characteristic length scale was found in relation to the nozzle length and width, based on the balance of the momentum flux to the buoyancy flux of the projected flame. A new non-dimensional heat release rate was defined based on the proposed characteristic length scale. The data obtained in this work for different aspect ratios, as well as those reported previously for axi-symmetrical sources, were shown to be well correlated by the derived function in two regimes: (1) for relative small non-dimensional heat release rates, the flame projection distance has a 2/3 power dependency on the heat release rate as for a 2-D trend fire; (2) for relative large non-dimensional heat release rates, the flame projection distance has a 2/5 power dependency on the heat release rate as for a 3-D trend fire. The proposed new correlation provides a more general and practical base for estimating the projection distance of horizontally oriented buoyant turbulent jet diffusion flames. (C) 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.