Metrology is a key factor in manufacturing and social life as well. One of the most common measuring equipment is flow meters, which can detect and measure the passing flow accurately. Diaphragm gas meters are simple tools that can measure the gas volume used in families. Nowadays, consuming gas is becoming increasingly popular in areas where it is difficult to use electricity, so designing and building a gas supply system is necessary. To manufacture a gas meter for each family, studying and calculating a sample of gas meters must be conducted. Based on the model of a diaphragm gas meter G1.6, this study calculated the error of this equipment to approximately +1%, based on some important parameters, such as ratio, gas volume, then proposed manufactured error permission of the valve, the part mostly decided the accuracy of the gas meter. Calculations were also verified by results from a testing process using the bell jar method. The findings of this research could be used as a reference for calculating, designing, and manufacturing a diaphragm gas meter further.