An ultrasonic flowmeter for measuring the flowrate of a fluid based on transit times of opposite propagating ultrasonic wave packets (21, 22), comprising: - a flow tube with a through-going opening for passage of the fluid between an inlet and an outlet; - two or more ultrasonic transducers arranged at the flow tube for transmitting and receiving the ultrasonic wave packets through the fluid; - a control circuit configured for operating the ultrasonic transducers to transmit and receive co-propagating and counter-propagating ultrasonic wave packets; wherein the control circuit is further configured to continuously determine the flowrate of the fluid based on sequential application of separate flow measurement sequences (fms) and flow estimation sequences (fes): - the flow measurement sequences (fms) including transmitting and receiving a co-propagating wave packet and a counter-propagating wave packet, digitally sampling the received wave packets and storing the sampled wave packets in a memory associated with the control circuit, determining a phase difference (Δφ) between the wave packets, determining the speed of sound (c) in the fluid, and calculating the flowrate (q) based on the phase difference and the speed of sound; and - the flow estimation sequences (fes) including alternately transmitting and receiving at least a co-propagating wave packet or a counter-propagating wave packet, digitally sampling the received wave packet and comparing the sampled wave packet with the corresponding sampled co-propagating wave packet or sampled counter-propagating wave packet, obtained during the previous flow measurement sequences, to determine whether a fluid condition, such as fluid temperature or fluid velocity, of the fluid has changed since the previous flow measurement. - a flow tube with a through-going opening for passage of the fluid between an inlet and an outlet; - two or more ultrasonic transducers arranged at the flow tube for transmitting and receiving the ultrasonic wave packets through the fluid; - a control circuit configured for operating the ultrasonic transducers to transmit and receive co-propagating and counter-propagating ultrasonic wave packets; - the flow measurement sequences (fms) including transmitting and receiving a co-propagating wave packet and a counter-propagating wave packet, digitally sampling the received wave packets and storing the sampled wave packets in a memory associated with the control circuit, determining a phase difference (Δφ) between the wave packets, determining the speed of sound (c) in the fluid, and calculating the flowrate (q) based on the phase difference and the speed of sound; and - the flow estimation sequences (fes) including alternately transmitting and receiving at least a co-propagating wave packet or a counter-propagating wave packet, digitally sampling the received wave packet and comparing the sampled wave packet with the corresponding sampled co-propagating wave packet or sampled counter-propagating wave packet, obtained during the previous flow measurement sequences, to determine whether a fluid condition, such as fluid temperature or fluid velocity, of the fluid has changed since the previous flow measurement.