High levels of energy consumption in buildings has emerged energy efficient design and optimization strategies. Due to high investment costs there is an increasing need for tools to accurately assess energy conservation measures (ECMs). Whole building energy simulations (BES) play an important role in energy efficient design and post occupancy assessment of buildings. In this study DesignBuilder and Ecotect, with an extensive use in both educational and architectural practice, have been used to model and simulate the energy consumption of an educational building. The results have been compared to on site audits. Both the energy consumption over a whole year and the thermal condition in indoor spaces for a day has been compared with simulation outputs. The study attempts to determine the main input parameters causing the discrepancies between building simulation and the real world performance. To validate and refine the simulation outputs, corrected inputs were applied. It has been concluded input parameters such as the infiltration rate, weather data, efficiency of cooling and heating systems and occupant behavior are the main causes of the discrepancies that should be considered carefully in simulations.