The Direct Evaporative Cooling (DEC) and the Indirect Evaporative Cooling (IDEC) systems have been increasingly proposed to be adopted as a dedicated fresh air system. Consequently, many manufacturers worldwide compete to provide leading edge technology of evaporative coolers with relativity high effectiveness for fresh air treatment. In the process of system selection by the designer or clients, the manufacturers submit their cut sheets that include measurement-based performance data at particular operating conditions along with annual performance data based on the equipment nominal performance, the project’s design and weather conditions. The annual performance prediction is evaluated by each manufacturer using in-house tools with different calculation concepts or methodology. This leads to diverse results in annual performance of equipment that has similar nominal performance from different manufacturers. Thus, it is of fundamental importance to establish a benchmark annul performance calculation tool to compare different manufacturers based on their nominal performance data and to verify their submitted annual performance data. Moreover, the benefit of having such standard tool is that it could be used by the designer for load calculation and system sizing during different design stages. Furthermore, the tool can be integrated in load calculation and energy modelling tools for thorough assessment of the sustainability effects of adopting evaporative cooling systems for fresh air treatment. This paper outlines an algorithm, applied via excel based VBA code, used to establish a benchmark performance for assessing these systems. The Algorithm use input of nominal system performance as the system wet bulb efficiencies that could reach up to 130% for some IDEC systems configurations (i.e. R-IEC and M-IEC). The input parameters include also the heat exchanger effectiveness for IDEC systems, weather file, the air flow rate, supply temperature and the project total cooling load. The output comprises of the total system operational hours, the water consumption and the resultant pumping load and associated annual energy. The algorithm can identify accurately the unmet load hours of the system; unmet hours represent the total hours when additional cooling coil is needed to achieve the targeted fresh air supply temperature. The tool has been validated and verified per ASHRAE chapter 19 and ASHRAE 140 analytical verification and comparative validation. Comparative validation results indicate that the tool output has only average 3% deviation. This paper will demonstrate a case study, of a major new commercial district in Cairo, Egypt in which the proposed benchmark algorithm has been used to compare the manufacturers data sheets provided by different suppliers. The fresh air treatment for this project is IDEC system. The tool will be used to verify the calculation results of multiple IDEC system manufacturers’ proposals for the project. The tool has efficiently identified the inconsistency in the manufacturers data and established decision matrix of which manufacturers provide the most reliable data for their products. The tool also has fully analyzed the system feasibility for the project with detailed output. Based on the case study results comparison, the algorithm presented in this paper is proven to be valid as performance comparative tool to identify best performing systems and detect any anomaly within the manufactures data. The algorithm can be integrated effectively in load calculation and energy modelling tools to be used by the designers.