Air quality in a classroom is very important. According to a study by Wargocki et al (2012) a shortage of ventilation in the classroom reduces the ability of students to concentrate and causes an increase in the number of errors during the elaboration of tasks. The aim of this study was to contribute to the optimization of acquisition and operating costs of HVAC equipment and to ensure hygienic requirements of air quality in the indoor environment for classrooms. An analysis of calculation methods to determine the optimal ventilation rate is carried out and the proposed methodology is compared to experimental results. Values of concentrations of carbon dioxide and ventilation rates are verified by experimental measurements. The methodology most suitable for calculating ventilation rate in Slovakia is selected based on the comparison with the results from experimental measurements. Based on this a methodology for calculating ventilation rates for classrooms is determined.