The invention relates to a measurement apparatus for measuring a flow rate of a fluid that flows with a main direction of flow in a circular line comprising an inlet section for conducting the fluid from the circular line into the measurement apparatus; an outlet section for conducting the fluid from the measurement apparatus into the circular line; a measurement section for connecting the inlet section to the outlet section; at least one ultrasound device for transmitting and/or receiving ultrasound waves, wherein the ultrasound device is arranged at a wall of the measurement section; and an evaluation unit for carrying out a time of flight difference measurement and for determining the flow rate, wherein the inlet section has a first superelliptic transitional shape and the outlet section has a second superelliptic transitional shape and the measurement section has a rectangular form, in particular having rounded corners.
