This paper studies one of the key aspects in the leak-before-break technique: the leak rate of a gas medium through penetrated cracks. The focus is on the results of theoretical studies. Based on the need for easy use in engineering calculation, two analytical models have been developed for the calculation of the leak rate of a gas medium. The first model is the isentropic flow model, which supposes that the gas flowing through the penetrated crack can be treated as isentropic flow. This model has theoretical solutions and can be applied easily, but it will overpredict the leak rate of a gas medium because it does not take the friction and other disturbances into consideration. The second model is the transient flow model, which only supposes that the gas flowing through the penetrated crack can be simplified as one-dimensional flow. This model can take the most influential factors of the gas flow into consideration. These factors include the variation of upstream pressure, various kinds of pressure losses during gas flow, and the heat exchange between the gas and the outside structures. The results of numerical simulations show that the transient flow model is suitable for engineering practices.