Apparatus and method for non-invasive measuring of the sound velocity of a fluid, such as a liquid, flowing in a tubing having points of two different and known transverse length has one sensor mounted at each point connected to a circuit that provides signals to each sensor that are returned to it after passing through the tubing wall and flowing fluid and reflection from the tubing internal wall opposing each sensor and from which the round trip transit time of the signals is measured and the sound velocity calculated from the two measured round trip transit times and the differential between the known transverse lengths. Flexible tubing is placed in the slot of a measuring head which deforms it to provide the two points at one location or the slot has two sections of different transverse length along its length with a point at each section.