An approximate solution is given for the prediction of the gas flow rate depending on the pressure drop for compressible laminar flow in circular ducts. The pressure and pressure drop range from high to low values, i.e. both Newtonian flow and Knudsen flow as well as the transition regime have to be modelled. The resulting equations are easy to handle and may be sufficient to serve engineering purposes. A numerical example displaying how fast a vessel can be evacuated is also included in the paper.