A method for determining a flow rate for a fluid in a flow tube of a flow measurement system, said system comprising at least three ultrasound transducer circuits, wherein each circuit comprises an ultrasound transducer arranged for transmitting ultrasound signals through said fluid in a transmitting phase, and for receiving transmitted signals from another of said at least three ultrasound transducers in a receiving phase, said method comprising the steps of: a1) exciting a first one of said at least three transducers to transmit said ultrasound signals at a certain frequency or a set of frequencies, in said transmitting phase, and reading out at least two non-transmitting transducers of said at least three transducers, in their receiving phase, and, determining a first time difference between two of said readings of said at least two non-transmitting transducers, b) determining a velocity of said ultrasound signals through said fluid based on at least said determined difference, thereby eliminating a transmitting delay of said first one of said at least three transducers, c) determining an attenuation in amplitude of said received ultrasound signals based on said read out of the at least two non-transmitting transducers of said at least three transducers; d) determining transmission coefficients for medium transitions occurring for said transmitted ultrasound signals between said at least two non-transmitting transducers based on said determined attenuation in amplitude, further comprising determining a velocity of a surface wave based on a time difference between said read out of the at least two non-transmitting transducers of said at least three transducers, and e) determining a mass density of said fluid based on said determined attenuation and said determined transmission coefficients for determining a mass flow rate of said fluid, characterized by f) determining a number of medium transitions for said transmitted ultrasound signals between said at least two non-transmitting transducers based on said determined velocity of said surface wave, wherein said surface wave is a Scholte wave, and wherein said mass density is further determined based on said determined number of medium transitions, said medium transitions comprising transitions of said ultrasound signals from said flow tube to said fluid and from said fluid to said flow tube, wherein the values for the transmission coefficients and the mass density are iteratively determined until a constant value for the mass density is obtained. a1) exciting a first one of said at least three transducers to transmit said ultrasound signals at a certain frequency or a set of frequencies, in said transmitting phase, and reading out at least two non-transmitting transducers of said at least three transducers, in their receiving phase, and, determining a first time difference between two of said readings of said at least two non-transmitting transducers, b) determining a velocity of said ultrasound signals through said fluid based on at least said determined difference, thereby eliminating a transmitting delay of said first one of said at least three transducers, c) determining an attenuation in amplitude of said received ultrasound signals based on said read out of the at least two non-transmitting transducers of said at least three transducers; d) determining transmission coefficients for medium transitions occurring for said transmitted ultrasound signals between said at least two non-transmitting transducers based on said determined attenuation in amplitude, further comprising determining a velocity of a surface wave based on a time difference between said read out of the at least two non-transmitting transducers of said at least three transducers, and e) determining a mass density of said fluid based on said determined attenuation and said determined transmission coefficients for determining a mass flow rate of said fluid, characterized by f) determining a number of medium transitions for said transmitted ultrasound signals between said at least two non-transmitting transducers based on said determined velocity of said surface wave, wherein said surface wave is a Scholte wave, and wherein said mass density is further determined based on said determined number of medium transitions, said medium transitions comprising transitions of said ultrasound signals from said flow tube to said fluid and from said fluid to said flow tube, wherein the values for the transmission coefficients and the mass density are iteratively determined until a constant value for the mass density is obtained.