Useful approaches to the calculation of gas flows in the molecular-continuum transition region have been presented by a number of authors. However they are restricted in application to particular conditions, such as long ducts or orifices with a large pressure difference, or have some theoretical limitation. A particular problem derives from the assumption of isentropic flow through an orifice. While a close approximation for high speed flows, the assumption leads to significant errors in nonchoked viscous flow. These errors then occur in transitional flow models which combine free molecular and viscous flows, particularly when these are applied to short ducts. In this article a simple model which overcomes some of the limitations of current models and enables calculation for all conditions of long or short ducts and large or small pressure differences through the whole pressure regime is presented. The results are found to agree well with published experimental data and with theoretically derived data for long tubes.