Flow number testing quantifies the permanent deformation characteristics of asphalt mixtures. For the flow number to be determined correctly, it is necessary that the material exhibits the three-stage creep response when tested. The current experimental protocol to be followed is not very clear for delineating such response. The post-processing algorithm for data analysis also plays a critical role in the determination of flow number. In this study, experimental investigations were carried out on an asphalt mix fabricated at air voids contents of 7% and 2%. The samples were tested at two temperatures (408C and 558C) and three confinement conditions (unconfined, 100 kPa and 200 kPa) for 100 000 cycles. Post-processing of the data was carried out using three different methods. It was found that the onset of the tertiary stage was strongly influenced by confinement conditions and air voids contents. The flow number determined by the different post-processing algorithms was different for all the conditions of testing. It is thus important for highways agencies to carefully select the testing conditions and post-processing algorithm before using this method for checking the rutting susceptibility of asphalt mixtures. © ICE Publishing: All rights reserved.