In a process and device for measuring a flow rate, the beam preferably emitted by a semiconductor laser (4) is split in the visible or infra-red range by means of a collimating lens system (5) and a twin hole diaphragm (6) connected downstream of the latter into two partial beams (1, 2) which are guided through the flow containing scattering centres. The total intensity of the partial beams or the reflected parts thereof passing through the flow is measured preferably by a photoelectric cell (7) and then amplified (8, 9). The autocorrelation function is derived from the measurement signal and processed (10, 11) in order to determine the transit time of the scattering centres through the partial beams. The process and device also yield satisfactory results in the case of high scattering particle densities and unknown flow profiles. Hardware costs are minimal and only a fairly small amount of data need to be evaluated, because only one receiving unit is required and only one measurement signal has to be evaluated.