A system and method for calculation of flow rate comprising at least two ultrasonic sensors, ultrasonic transmit receive device, and signal processing device. At least two ultrasonic sensors are installed on the pipe where the fluid is flowing through, and at least two ultrasonic sensors contain different ultrasonic beam paths, further, the ultrasonic beam paths of at least two ultrasonic sensors contain the overlap area. The ultrasonic transmit and receive device is used to actuate at least two ultrasonic sensors, and is used to transmit one or multiple ultrasonic signals to pipes via one or multiple of at least two ultrasonic sensors, further, to receive ultrasonic signal via at least two ultrasonic sensors. The signal processing device is used to process the ultrasonic signal received, further, to calculate the flow rate of the fluid precisely.