An analytical, three-dimensional, steady-state thermal model of a high-power diode laser bar is presented in this paper. The heat spreading angle in a laser bar heat sink, subjected to several convective conditions on the bottom-side, was calculated with this model. Thermal design curves for the heat sink and submount are also presented. Special discussion is presented for two kinds of our conduction-cooled laser bars. Finite element simulation and experimental results based on the wavelength shift method are compared with this analytical solution. The familiar 45 degrees angle in thermal design for a commercial hard solder conduction-cooled laser bar was found to lead to a 12% increase in thermal resistance relative to a free lateral diffusion heat sink. (C) 2018 Optical Society of America