The method of accurately measuring the volume of urine in a bladder includes generating a plurality of ultrasound scan planes using the received ultrasound signals; detecting distances between the front and back walls of the bladder for each scan line in the ultrasound scan planes; determining an area of the bladder using the detected distances of the scan lines for each ultrasound scan plane; generating a virtual radius of the bladder using the determined area of the bladder for each ultrasound scan plane; determining a calibration coefficient for each ultrasound scan plane using the detected distances of the scan lines; calibrating the virtual radius of the bladder for each ultrasound scan plane using the calibration coefficient; determining the volume of urine in the bladder by calculating the volume of a sphere of which the radius is the average of the calibrated virtual radii for the ultrasound scan planes.