A method for determining a flow rate, Φ t, of a fluid in a conduit, the method comprising: providing a first ultrasonic transit time signal (115, 204, 314) under fluid flow, generated using a first transducer in a direction towards a second transducer; providing a second ultrasonic transit time signal (125, 206, 316) under fluid flow, generated using the second transducer in a direction towards the first transducer, the first and second transducers (110, 120) being spatially separated from one another along a length of the conduit; providing a third ultrasonic transit time signal (200, 310) under zero fluid flow, generated using the first or second transducer (110, 120) in a direction towards the second or first transducer (120, 110) respectively; generating respective measures of amplitude and phase of the first (115, 204, 314), second (125, 206, 316) and third signals (200, 310) at a selected time to provide respective first, U, second, D and third, Z, flow vectors; calculating a measure of the difference in the values of amplitude and phase of the first signal (115, 204, 314) and second signal (125, 206, 316) at the selected time using the first and second flow vectors (414, 416) to provide a difference vector, Δ; generating a measure for a component, m y, of an interference vector, M, representing a conduit-borne component signal of the first and second signals (115, 125) using the first and second flow vectors (414, 416) and the difference vector, Δ; and generating a measure for the flow rate, Φ t, of the fluid using the component, m y, of the interference vector, M, the third flow vector, Z, and a sum, Σ, of the first and second vectors (414, 416). providing a first ultrasonic transit time signal (115, 204, 314) under fluid flow, generated using a first transducer in a direction towards a second transducer; providing a second ultrasonic transit time signal (125, 206, 316) under fluid flow, generated using the second transducer in a direction towards the first transducer, the first and second transducers (110, 120) being spatially separated from one another along a length of the conduit; providing a third ultrasonic transit time signal (200, 310) under zero fluid flow, generated using the first or second transducer (110, 120) in a direction towards the second or first transducer (120, 110) respectively; generating respective measures of amplitude and phase of the first (115, 204, 314), second (125, 206, 316) and third signals (200, 310) at a selected time to provide respective first, U, second, D and third, Z, flow vectors; calculating a measure of the difference in the values of amplitude and phase of the first signal (115, 204, 314) and second signal (125, 206, 316) at the selected time using the first and second flow vectors (414, 416) to provide a difference vector, Δ; generating a measure for a component, m y, of an interference vector, M, representing a conduit-borne component signal of the first and second signals (115, 125) using the first and second flow vectors (414, 416) and the difference vector, Δ; and generating a measure for the flow rate, Φ t, of the fluid using the component, m y, of the interference vector, M, the third flow vector, Z, and a sum, Σ, of the first and second vectors (414, 416).