Abstract: A system or a method for measuring flow of fluid or gas, has a flow duct with at least two transducers which generate at least one beam of ultrasound in the flow duct in which ultrasound transducers are placed in a fixed positions in a duct for flowing air where the electronic device for analyzing signal from the transducers is performed in a handheld device. A transmitter circuit is permanently connected to the transducers and a receiver circuit is placed in a handheld device. The transmitter circuit has calibration data representing the actual placement of the transducers in relation to the actual duct, which calibration data is communicated by every connection to the receiver circuit. Hereby can be achieved that transducers can be permanently placed in ventilation ducts in buildings, where placement of ultrasound transducers for measurement and for calibration of measurement of an air stream is nearly impossible.