In this work, a model of gas leakage through nonmetallic gaskets was developed in order to predict leakage rate of gasket sealing connections. The model was verified by the leakage experiments on two types of gaskets: compressed non-asbestos fiber gasket and flexible graphite gasket reinforced with tanged metal sheet. The coefficients in the leakage rate formula were obtained by regression of experimental data for each type of gasket. The model was also validated against the experimental leakage data by other researchers and shown to produce accurate predications. Furthermore, the model was applied to a bolted flanged connection in service in order to assess the tightness of the connection.