Useful approaches to the calculation of gas flows in the molecular-continuum transition region have been presented by a number of authors. These have covered particular conditions such as long ducts or orifices with a large pressure difference for example. There is, however, no generally applicable method which enables calculation under all conditions. A particular area of difficulty is the isentropic-molecular transition. The paper presents a simple model which enables calculation for all conditions of long or short ducts and large or small pressure differences. Results are found to agree well with published data.