A method for determining an inner diameter of a sounding tube, which, for measuring the fill level of a fill substance located in a process space of a container, extends in the process space, or is placed alongside the container and connected with the process space. The method can be implemented in the case of a fill-level measuring device working according to the FMCW-principle. Besides the intermediate frequency of the difference signal, also its phase shift is ascertained, wherein the exact tube inner diameter can be determined based on the phase shift. An advantage of the method is that the fill-level measuring device with the help of the then exactly known tube diameter can be recalibrated and accordingly the fill level determined more exactly. The exact tube inner diameter does not have to have been previously known.