About half of the energy loss in buildings is wasted through windows. Determining the optimum window-to-wall ratio (WWR) for different building facades would reduce such energy losses. The optimum WWR is the window area that minimizes the total annual energy of cooling, heating, and lighting. The purpose of this study is to investigate the optimum WWR of different facades of an office building. For this purpose, a sample building is simulated by means of DesignBuilder software in order to investigate the annual solar heat gain, cooling load, heating load, and lighting consumption for the three cities of Bushehr, Shiraz, and Tabriz, and optimum window areas of office buildings for the three cities are determined. Based on the results, the optimum window area for the north building facade for all climates is 20–30%. This amount for the southern facade of the building in Bushehr, Shiraz, and Tabriz is, respectively, 20–30%, 10–30%, and 20–50%. The optimum window area for the eastern and western building facades in Bushehr is 30–50%; in Tabriz it is 40–70%, and in Shiraz it is 20–60% and 40–70%, respectively. The difference between the maximum and minimum energy consumption with different window areas in Bushehr and Shiraz is 20–100% and in Tabriz it is 16–25%.