An ultrasonic flow meter arranged to measure a flow rate of a fluid, the flow meter comprising: - a flow tube (F) with a through-going flow passage for passage of a fluid between an inlet (I) and an outlet (O), and two through-going openings (P1, P2) arranged in a wall of the flow tube (F), each one for providing an opening into the flow passage; - a house (H) separate from the flow tube, formed by a monolithic polymer house element having a bottom part (B) and a side part (W) to form a cavity (C) with a single opening, wherein the bottom part (B) has two areas arranged for transmission of ultrasonic signals, said areas provided with protrusions (PA1, PA2) arranged to enter the two through-going openings (P1, P2) in the wall of the flow tube (F), and wherein the house (H) is arranged for being mounted on the flow tube (F) so that the protrusions are arranged in the openings (P1, P2) into the flow passage; - two or more ultrasonic transducers (T1, T2) arranged in the cavity (C) aligned with said protrusions (PA1, PA2) and arranged for emission and receipt of ultrasonic signals through the areas of the bottom part (B) of the house (H); and - a mounting element (M) protruding from the house (H) boundary, said mounting element (M) being arranged to reinforce the bottom part of the house, the mounting element being arranged in mutual engagement with the bottom part of the house, and where the bottom part (B) of the house (H) and the mounting element collectively absorb forces from a fluid pressure in the flow tube when the house is mounted on the flow tube, characterized in the mounting element (M) being plane and having through-going holes (HS1) to allow fastening by means of screws to the flow tube (F). - a flow tube (F) with a through-going flow passage for passage of a fluid between an inlet (I) and an outlet (O), and two through-going openings (P1, P2) arranged in a wall of the flow tube (F), each one for providing an opening into the flow passage; - a house (H) separate from the flow tube, formed by a monolithic polymer house element having a bottom part (B) and a side part (W) to form a cavity (C) with a single opening, wherein the bottom part (B) has two areas arranged for transmission of ultrasonic signals, said areas provided with protrusions (PA1, PA2) arranged to enter the two through-going openings (P1, P2) in the wall of the flow tube (F), and wherein the house (H) is arranged for being mounted on the flow tube (F) so that the protrusions are arranged in the openings (P1, P2) into the flow passage; - two or more ultrasonic transducers (T1, T2) arranged in the cavity (C) aligned with said protrusions (PA1, PA2) and arranged for emission and receipt of ultrasonic signals through the areas of the bottom part (B) of the house (H); and - a mounting element (M) protruding from the house (H) boundary, said mounting element (M) being arranged to reinforce the bottom part of the house, the mounting element being arranged in mutual engagement with the bottom part of the house, and where the bottom part (B) of the house (H) and the mounting element collectively absorb forces from a fluid pressure in the flow tube when the house is mounted on the flow tube, <EOS>