An ultrasonic flow meter arranged to measure a flow rate of a fluid, the meter comprising - a housing, wherein the housing comprises a flow tube defining a flow channel with first and second fluid openings (O1, 02) at each end of the flow tube; - first and second ultrasound transducers arranged to transmit and receive ultrasonic signals through a measuring tube via first and second ultrasound reflectors (R1, R2) fixed at respective ultrasound reflector holders (RH1, RH2) arranged inside the flow tube, so as to allow ultrasonic measurement of a fluid flowing through the measuring tube; wherein: - a first mounting face for mounting of a first flow insert is formed in the flow tube by a reduction in a cross-sectional area of the flow channel at a first distance from the first fluid opening; - a second mounting face for mounting of a second flow insert is formed in the flow tube by a reduction in a cross-sectional area of the flow channel at a second distance from the first fluid opening, the first distance being larger than the second distance; - the first flow insert is made of a polymeric material and forms a first ultrasound reflector holder arranged outside a first end of the measuring tube and a mounting flange being arranged for engagement with the first mounting face; and - the second flow insert is made from a polymeric material and forms a second ultrasound reflector holder structurally connected to a mounting flange and being arranged for engagement with the second mounting face, characterised in that: - the housing is of a polymeric material; - the cross-sectional area of the flow channel at the first distance is smaller than the cross-sectional area of the flow channel at the second distance; and - the first flow insert forms a measuring tube and has a mounting flange structurally connected to the measuring tube. - a housing, wherein the housing comprises a flow tube defining a flow channel with first and second fluid openings (O1, 02) at each end of the flow tube; - first and second ultrasound transducers arranged to transmit and receive ultrasonic signals through a measuring tube via first and second ultrasound reflectors (R1, R2) fixed at respective ultrasound reflector holders (RH1, RH2) arranged inside the flow tube, so as to allow ultrasonic measurement of a fluid flowing through the measuring tube; - a first mounting face for mounting of a first flow insert is formed in the flow tube by a reduction in a cross-sectional area of the flow channel at a first distance from the first fluid opening; - a second mounting face for mounting of a second flow insert is formed in the flow tube by a reduction in a cross-sectional area of the flow channel at a second distance from the first fluid opening, the first distance being larger than the second distance; - the first flow insert is made of a polymeric material and forms a first ultrasound reflector holder arranged outside a first end of the measuring tube and a mounting flange being arranged for engagement with the first mounting face; and - the second flow insert is made from a polymeric material and forms a second ultrasound reflector holder structurally connected to a mounting flange and being arranged for engagement with the second mounting face, characterised in that: - the housing is of a polymeric material; - the cross-sectional area of the flow channel at the first distance is smaller than the cross-sectional area of the flow channel at the second distance; and - the first flow insert forms a measuring tube and has a mounting flange structurally connected to the measuring tube. - the housing is of a polymeric material; - the cross-sectional area of the flow channel at the first distance is smaller than the cross-sectional area of the flow channel at the second distance; and - the first flow insert forms a measuring tube and has a mounting flange structurally connected to the measuring tube.