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Objective: Humans produce and exhale CO2, thus the concentration of this gas increases in closed environments. The CO2 concentration of air is often used as a reference to measure the ventilation rate. The typical outdoor CO2 concentration is approximately 400 ppm, although it can be as high as 500 ppm. Concentrations greater than 20000 ppm result in deep breathing, higher than 100000 ppm cause visual disturbances and tremors with possible loss of consciousness and over 250000 ppm may cause death. In buildings with no change on their ventilation rate, high CO2 concentrations have negative effects on decision making and working performance. At 1000 ppm, performance is significantly reduced in six of nine decision-making metrics compared to 600 ppm. In this work, a CO2 flexible monitor is designed to measure ventilation in closed environments. Methodology: Electrolytic and infrared CO2 sensors with a detection range of 350 to up to 10000 ppm were used. The used sensors have good sensitivity and selectivity to CO2. The gas monitor has a simple calibration system, whereby softwareautomatically adjusts the calibration curve parameters after circulating clean air. The design of a gas bench used to verify sensor calibration is also shown. Results: A set of measurements were performed with electrochemical gas sensors and infrared (IR) gas sensors to test the functionality of the equipment. Experimental work has shown sensors have a satisfactory response for this application. The margins of error are +5 % of the reading value. Conclusions: A low cost, flexible gas monitor for indoor environments like schools, offices, laboratories, and industries was designed in this work. Due to the flexible design, a network of gas monitors strategically distributed in the different spaces of the buildings is proposed. Fundings: Universidad Tecnológica Nacional. Comisión Nacional de Energía Atómica. Buenos Aires, Argentina